Alex, some wise words. One thing I have learnt is to be up front with HMRC. When I started down this road I had a very fruitful conversation with a chap in Glasgow. We clarified the nature of my work, allowances, tax relief opportunities for further training, employment status etc. I also piggy-backed my wife's accountant for book-keeping and this has been well worthwhile. I invoice the flying clubs/schools I work for, I keep all receipts and I declare absolutely everything. It is my experience that if one engages with the Revenue, they are extremely helpful. They only become a pain in the bum when they feel you are intentionally trying to avoid them. If someone has been self-employed for a decade or so, has only worked at Timbuktu Flying Club (five days a week) and has never filled in a tax return then ............
PS. To anyone who is instructing self-employed, make sure you have the right level of insurance sorted out.