Hoya Pro-1D UV digital filters from £18.49.
Seems a lot cheaper than anywhere else. I picked up the £26.99 67mm version for my 70-200 L. Add £3.00 p&p for Royal Mail. Arrived next day :)
They appeared to be £35 upwards elsewhere. Should be able to pick up in store to avoid the P&P but you might have to ask them to match the internet price.
Direct link is to Filters section of site. Choose Hoya from search options.
Yes, you can get cheaper filters, and yes, some people may ask why a piece of glass is so expensive, but those that know will understand.