In reply to Mooncat:
Indeed - the cost of new qualifications isn't claimable. Oddly, the cost of maintaining old ones is, so you can deduct your first aid ticket renewal costs every three years.
And you can claim part of the cost of equipment you already owned, as long as you use it to earn with. I'd have to dig up my last tax return to check, but I think I state that 25% of the use of my rack, ropes, and PC is business related so I can claim a certain (annually decreasing with depreciation) percentage of that back.
Just call HMRC; despite their reputation, they answer the phone very quickly (as long as it's not the day before SAs are due!) and are really helpful.