In reply to TommyH:
Firstly, and I know this seems like nit-picking, but you're not 'claiming anything'. What happens is, you spend money which wholly relates to the business in which you deal. That expenditure reduces your tax liability. Should you end up paying more tax then necessary, you receive a rebate. I
t took an accountant to get my head around this - don't spend money to avoid paying tax, because you pay anyway and you don't ever really have much money to save. This of course depends if you have any grand plan to save for the future, mortgage etc.
As to the question, if the money spent, wholly relates to your business, then it's deductible against tax. It can get murky about retraining for things that are not your normal line of business though, as the tax man sees that as capital investment (apparently).
I assume you're an instructor. I'm not a watersport coach. If I trained to become one HMRC could take the line this falls within my current role as an instructor, thus it would be deductible. They could also take the line that it's a new skill, thus capital investment and so a different kettle of fish. Two advisors have separately given the above advice. They also said that since I'm not a big earner and if I don't take the p*ss then I probably have nothing to worry about.
They also advised against taking advice from randoms on the Internet.