In the cold, windswept tunnels beneath the royal parks they stand, in single file with mop and bucket, their heads bent, faces set in stony disconsolation.
They are a 21st-century chain gang reluctantly returning the lustre to the shabbier corners of the royal wedding route.
Over their blue boiler suits are DayGlo orange jackets with the words "Community Payback" emblazoned on the back, neatly offsetting their bright yellow marigolds.
"We set up this scheme because we wanted get the place sorted out and looking nice for the royal wedding," said Lisa Houslin, the Community Payback scheme manager.
Over the next four weeks, offenders with convictions ranging from driving offences to public disorder will clean the pedestrian tunnels beneath Hyde Park Corner, a key thoroughfare for the expected millions who will arrive from all corners of the globe to witness the nuptials of Prince William and Kate Middleton.
"I don't want to be doing this," said Robert Hall, a 21-year-old from Ladbroke Grove in West London, currently 17 hours into his 100-hour sentence for theft and handling stolen goods. "Last week, I was painting railings on the Mozart Estate, doing something that actually needs to be done, in a deprived area, doing something useful, making the estate look better."
Police struggled to break up a 60-man brawl on the rundown Mozart Estate three years ago, resulting in a plea from local MP Karen Buck to "act now before someone gets killed". It is a situation unlikely to replicate itself at the Lanesborough Hotel, the entrance to which can be found at the tunnel's exit. Its royal suite is the capital's most expensive hotel room.
"These tunnels get cleaned all the time," said George, 21, who has a conviction for aggravated theft. "This is a nice area."
Beneath his sponge is a mural of Catherine of Brunswick and chums cavorting in the sunshine, dipping their toes in the Serpentine and having their feet towelled by the grateful hoi polloi. "I would rather be doing something more private than public," he said. "I go to college in this area, I don't need people seeing me doing this, wearing my orange bib."
Standing between them and the passing public - it is a pedestrian bypass almost as busy as the roundabout above - is their supervisor, Errol Wallace, a human being of a size probably unimaginable when the cramped tunnels were laid just more than 100 ago. On the mural next to him, the Duke of Wellington's armies are rampaging on horseback at the Battle of Waterloo.
"I make sure they do what they have to do, there's never any fuss," he said. "But when they've finished, it'll be good that they can be proud of what they've done and their input in this huge world event."
Pride is a not common sentiment among his charges. "You can't get away from this wedding can you? I won't be watching it on the telly," said Jimmy, 37, originally from Brazil. He has served 50 of his 200 hours, but refuses to disclose his misdemeanour.
"There are other areas, other communities that could do with help far more than this one," he said. "I would rather be painting play areas. There's parts of Clapham Junction where I live that need help much more than this. But cleaning here doesn't hurt anyone. And it's important that we do our payback."
Other Community Payback schemes include gardening, cleaning graffiti and picking up litter. Thousands of payback hours went into clearing snow from roads and footpaths last winter. Houslin insists resources have not been diverted specifically from other projects to prepare for the royal wedding.
One person who wouldn't mind if they had been is Youssef Chaban from Lebanon.
"I don't think William and Kate will see our work," he said. "They probably won't be passing through Hyde Park tube on the big day, but lots of tourists will.
"I hope they enjoy it."