tayto2 - there are no definite rules from Revenue on this- not even good guidance.
However, if you intend to continue what you are doing for some foreseeable time ,and if you can't afford a good accountant, then -strange as this may seem - you can actually ask Revenue.
I've done this a few times when I've submitted my self assessment. (Mind you I didn't leave it as late as you.) Only once did they disallow something (quite small, and I half-expected it).
I honestly told them that I was not sure about this-or-that and expense/income but I put it down and said sorry if I am not correct etc etc grovel grovel. On the Self assessment form there is a box you tick when you wish to express doubt about something, and you enclose your letter expressing doubt with the form.
So, submit what you think are reasonable costs and see what happens.
Sadly, the advice above about minimising your expectations is valid.
Anyway, you won't be shot if you ask them and if you don't go bonkers on claims.
But get that form in now !