I did the private Sea Plane rating written test with Alan Newton at Ontrack at Wellesbourne and scored 100%. He does a days training with the test at the end and it is very very good value for money. I went and sat the Professional exams a week later at Gatwick and got 96% based on what I remembered from his days training.
I was transferring a commercial sea plane rating from my FAA certificate. To add the rating to your JAR licence you then have to demonstrate that you have 5 hours of dual training and at least 12 take off and landings. The CAA wil then relieve you of £84 and add the rating with test exempt and an expiry 2 years hence. To revalidate you have to do 12 take off and landing in water in the last 12 months and then everything else is done at the same time as your SEP.
Their is an error in LASORS that says you must do a skill test which is not the case. If you do the test with Alan he will help out if any problems.
PM me if you need more info.