Hi James.
I revalidated my ppl at stapleford flight centre earlier this year and would recommend them. The reception and admin side of things is a little chaotic and disorganised at times but the instructors I had were all excellent. They also have a fleet of c152s which are cheaper than other types you may be offered, and no landing fees are charged which adds up to a huge saving when you consider some airfields charge £25 plus per landing.
It's a radio field with a very busy circuit which is intimidating at first but definitely good experience. If you can cope with the stapleford circuit you can cope with anything!
I also live near London bridge and found the journey very easy - northern line
to bank and then central line to Debden. The airfield is a short cab ride from the station. I would say visit a few schools and get quotes - although watch out for hidden charges like landing fees and fuel excess charges. Whatever you do don't pay upfront for training as some high profile schools have gone bust over the last couple of years!