If I'm buying new parts I go to Fowlers in Bristol, they know what they're doing, they're helpful and have a surprising amount in stock. Much better than any of the local dealers I've found and even the independant garages around here use them. Have your chassis no. to hand when you phone them though, the whole number not just the 3 digit pre-fix.
http://www.fowlers.co.uk/
I'm not affiliated with them in any way, I just tried a lot of rubbish places first.