How do you feel about your partner's ex-girlfriend? A bit suspicious, probably. Perhaps threatened. Most likely you try not to think about her.

How would you feel if she lived only a few streets away and you shared the same high street and coffee shops? How about having dinner with her? What about a joint holiday? Sound crazy? Welcome to my world.

My husband's ex-girlfriend is not just a friend, but my best friend. Previously, I have been as obsessive and unreasonable about boyfriends' exes as anyone - finding their mere existence a threat to my relationship.

I have been known to go berserk when I've found forgotten photos of exes tucked into books or drawers. I've demanded all evidence of previous girlfriends be thrown out. And I've wondered whether a boyfriend can truly love me if he once loved someone else.


Then I grew up and realised that life and love aren't really like that.

When my husband, Giles, and I met, he was 38 and I was 27. We both had a Rolodex-worth of exes in all shapes and sizes. Nice ones, mean ones, gay ones, unfaithful ones. We talked about them freely and there was never any pretence that we didn't both have histories.

We had both been through the relationship mill - we were exhausted. The difference was that whereas I usually pretended my exes were dead, Giles kept in touch with his. There was Anna, the first serious girlfriend, then Katie, the long-term girlfriend - they were together for eight years, so she is more like an ex-wife - and then Melissa, the one who got away.

Melissa quite literally fled in the middle of the night following a series of rows, ending their four-year relationship. But by the time I came along four years later, that was all ancient history. She and Giles chatted on the phone and often met up for tea or lunch, as if they were old friends rather than ex-lovers.

Should I have felt threatened?

On reflection, I could have felt horribly threatened. They had their own private language that they still used: "Ho, Scrams," Giles would say in greeting when she rang. "Steaming" was their word for tea, "rumlings" were children. Even the house Giles lived in when we met, now our family home, he had bought to live in with Melissa - before she thought better of it and did a runner.

There were photos of her dotted about here and there, in albums and photo-collages. On a shelf was a little toy Piglet, which Giles had given to Melissa - or was it the other way around? I can't remember. But she was everywhere. She even lived nearby. But for some reason it never bothered me. Perhaps her being there in plain view made her feel less threatening.

Usually, with only scant detail to go on, you picture the ex as a seductive temptress. But with Melissa there was no opportunity for fantasy. She was right there - a normal person, just living her life.

In fact, the more I learnt about her, the more similar we seemed to be: both Londoners and writers, neither of us very sociable or super-ambitious. If that had been the extent of it, we would probably never have become friends. But something else drove me to make that extra effort.

Giles had been in two two-year relationships since splitting from Melissa, and both girlfriends had taken offence to her. They had forbidden Giles to see or talk to her (but he did anyway) and it had caused awful rows. So out of sheer competitive spirit, I was determined that not only was I going to be completely cool with Giles remaining friends with Melissa, I was actively going to be friends with her myself.

Also, it was clearly non-negotiable that he and Melissa would remain friends, so I went along with it.

I first met Melissa seven years ago, for lunch at her house with Giles and her husband Peter, six months into our relationship. I can't remember what I was expecting but it wasn't the petite and smiley, chatty and funny, interested and interesting person I found.

She buzzed about, fizzing with stories and ideas. She was a mash-up of Titty from Swallows and Amazons, the Queen, your favourite auntie and the slightly eccentric owner of a rare-book shop. At first I was baffled as to what this lighthearted little ray of sunshine and my dark, brooding boyfriend could have possibly seen in each other, but then opposites do attract.

"This is going to be very easy," I thought as we nattered away to each other. We were both trying to redecorate our houses, so we rambled on about the great value of Topps Tiles ('You'd never know they were 49p each!') and how much we love John Lewis.

Having children brought us closer

Although we liked each other well enough, we did not become best buddies straight away. When Giles and I got married two years later, for example, we didn't invite Melissa and Peter. (Not out of animosity, it wasn't a very big wedding.) But then we both had children around about the same time. Melissa and Peter had Alec in 2009 and My in 2012, and I had Kitty in 2010 and Sam in 2013.

We were often confined to the house, with only phones and laptops for company, and struck up a sort of pen-friend relationship over email, despite living so close to each other. Driven partially insane by having two children under three, we quickly threw off all niceties and started sending each other emails with subjects such as "Delete after reading!".

There were mad, sleep-deprived rants about our husbands or children, details of crazed crushes on local GPs, and miracle cures for infant maladies.

There was more. My husband can be difficult in a way that can be hard to explain. I don't want anyone to think that my marriage is in trouble, because it isn't, and I don't want anyone to think the less of Giles. But from time-to-time we have fights that I want to talk about, but not to anyone who will worry or judge.

In Melissa, I have someone who completely understands all my husband's eccentricities and quirks. I don't have to spell out certain things; she already knows. When Giles and I argue about how much telly I let the kids watch, Melissa will sympathetically listen to me grumble about how husbands don't understand how hard sole-charge childcare is.

If she secretly relishes hearing tales of marital discord between her ex and his wife, she never lets on. We don't discuss the past, neither do we discuss our sex lives, because neither are relevant to our friendship. In turn, Melissa has ended up marrying someone with plenty in common with Giles - hot-headed, ballsy, prone to brooding, obsessed with cricket and totally unafraid of any confrontation anywhere about anything.

Happy holidays

They are also both hard workers and good husbands. One year, Giles proposed that we all go on holiday together and Melissa and Peter agreed. I had no problem with it, I didn't think it was weird. They didn't think it was weird either. Maybe we were all the right kind of weird together.

Anyone I mentioned the arrangement to was horrified and looked at me askance. They immediately inferred some kind of strange goings-on - perhaps we were swingers? (We are not.) It could have been a disaster. When we arrived at the villa there was a dead lizard on the doorstep, an abandoned slice of toast under the grill, gritty sheets and thin towels - not to mention an empty, very deep Jacuzzi that was effectively a toddler death trap.

The bottom of the pool was strewn with leaves. It was a tough test for any friendship, but we all pulled together. Giles got on the phone to find us a new villa, Melissa and I pottered about with the kids trying to keep calm, and Peter, who is a lawyer, rang the villa owner to calmly negotiate a refund.

In the end, it was a lovely holiday and next year, we are going away again, to Corfu. I can't wait. There are, I guess, unspoken rules. Melissa doesn't talk about what went on in her relationship with Giles and I don't ask her about it. I know from ancient conversations with Giles that he was a bit of a mess, emotionally, while he was in the relationship and feels regretful about being moody and elusive. Beyond that, the less I know the better, really.

Melissa never crosses the line from friend to ex. She is never proprietorial, never over-familiar or knowing - and neither is Giles. I suppose what it boils down to is that Melissa being Giles's ex-girlfriend is entirely incidental. We would most likely have met at some point anyway, seeing as we live near each other and have children of similar ages.

Friendships can't be forced or faked, but neither should you write one off just because of circumstance. I'm certainly pleased I didn't.