Have the bit of paper stashed somewhere that tells you the CAA requirements...
As far as I can remember you have to do your 5 hours, pass the flying test, do a 'seamanship' test (or some word like that) which is multi choice and costs £37 ish, then apply for the rating by sending in a huge wodge of cash and your logbook. I decided not to, but to keep my rating as an addition to my US PPL (it seems you CAN add ratings to a licence issued on the basis of a UK PPL!) and do my flying out in the states and Canada. It's a bit academic though as the insurers over there won't let you near a seaplane on your own without loads of experience. I can recommend a man in Quad Cities, Illinois for his C180 course - lots of landings as there is no need to fly round a lake to get back to the water - the mississippi just keeps on going.... Would love a go on the Seabee though - do you have any contact details or a company name?
A quick call to the CAA will be all it takes for them to send you the same bit of paper they sent me - can't find it on their website.
J-R