The Last Chance Saloon on our Monopoly circuit is Make general repairs on all of your houses. Obviously we're going to substitute bikes for houses, and talk about cycle repairs.
My experiences of bike repairs and servicing in London have been mixed. I've only had good quality annual services or one-off repairs from two shops, Apex Cycles in Clapham and the London Bicycle Repair Shop (right) in Hatfields, near Waterloo. Too many others, especially chainstores, have been barely adequate or downright bad.
There's a useful list of bike shops, including repair and rental, on the Southwark Cyclists website. They recommend Druid Cycles (formerly i-Bicycle), a new place in the railway arches in Druid St. It apparently offers a courtesy bike for you to use while yours is being fixed, a necessary service surprisingly lacking in most bike shops.
Anunal services don't work out cheap - I reckon on £100-£200 per annual service, because the 10,000 miles or so I do a year means regular replacement of chainring, chain and rear cassette (and you usually have to replace all three at once, or else the mix of new and old makes the chain skip and jump, and bikes are not meant to be marsupials).