London Stone are bad, but to be fair, I suppose they aren't any different than other London letting agencies. Very helpful and friendly until the contract is signed, and after that the tune changes dramatically. Virtually impossible to get any maintenance done and they will do and say anything to get out of doing any actual work. I chose not to renew my contract after a year, and seeing how they make more money out of a new tenant, that is probably the outcome they were looking for. Or perhaps they are just being indifferent. But again, this may well be how agents in London operate.
My advice to anyone wanting to rent a flat through London Stone (or any other agent for that matter) is to always, always make sure you have everything in writing and take plenty of photos of everything and everywhere in your flat/house. They will try and pin every conceivable (and inconceivable) cost on you when you leave.
As a landlord I would stay away altogether though. In the particular flat I rented there were several problems that needed attention. Of course the agent promised that this would all be taken care of (again - get these things in writing), but a year and many reminders later they were still being ignored. This despite the fact that these were problems that left unattended would eventually lead to costly repairs.
There are many other agents in the same area who will be advertising the same flats, so there is no reason to go with London Stone. Then again, they may be just as bad so check the reviews. I can only review LS and advise against.