Just as a note, I find it really interesting that someone like Balfe's bikes; who only has three outlets tips into a 'oh no, they're TOO successful'; a one man band did well and expanded to a shop, then to two outlets; then to three.
Why do we collectively only value individual success and have such a narrow definition of independents. Do we block Sally White's now she's extended from one coffee shop to two because that's something close to corporate. (I hope not, her cakes are scrumptious)...
I think it's more that we misunderstood the original question. I didn't know there were people who worked as mechanics, outside of bike shops. I wouldn't even know where to start to look for someone like this!
Dr NipNip (http://www.nipnip.co.uk/) may fit the bill - they are a mobile bike mechanic service who come to you. According to their website they are based in SE1 so you could probably go to them, too.
They used to come by my old office and I once had my headset serviced. It was convenient not having to go to a bike shop and they did fix the problem for a reasonable price.
However for servicing these days I tend to go to Swift Cycles in Spitalfields. Always been pleased with the results and they do take a lot of care with your bike. Only problem is the workshop is often booked up months in advance.