I've bought all my replacement valves from HotRox, and quite a few other bits as well...
I've always found them to be spot on price wise, friendly on the phone, quick delivery, and just a general, all round good service really. Can't fault them. Their prices for EHX pedals can't be beat... IMO.
I ordered 6 12AX7's once, and was only sent 5, rang them up, they apologized & promised it was going out that day, I received No.6 the next day...
I think Watford's charge more because they do have all the testing rigs etc, and do undertake a lot of testing/burning in/matching on site, as well as carrying a huge range of different valves.