RMA never sign for their letters, they are a complete nightmare to deal with and NEVER listen to anything you have to say.
Try this for their address:
Old Docks House, Watery Lane, Preston, England, PR2 1AU
Its for NCO, but as they are the same company doesn't really matter.
I ended up sending my CCA there, and luckly they signe for it.
Oh well they are now in non-compliance and digging themselves deeper in the smelly stuff as I'm working with Trading Standards about their operation.
Also NEVER talk to them on the phone.
They will swear blind that the sky is RED just to get you to agree to anything.
Also it's worth talking to BT and either getting Choose to Refuse or even better changing your number, this is the only sure fire way I found to stop them calling.
I had 6 weeks where they called upwards of 5 times a day !!!!
They still haven't written to say they need my new number, but they have said they are sending my account to their "paramilitry Wing"

Now that will be fun, I can't wait.
Also LOG EVERYTHING in your deals with tme and report them to TS.
TS I have found are really helpful and completely non-judgemental, I'm sure Dave will say the same.