if I put £500 pm aside, then paid for fuel and oil out of a separate amount, that *should* be OK
That must depend very much on where you are, and I understand the UK is not the most favourable place for light private aviation.
500 sterling per month might amount to some 5000 € yearly for me, more than enough to cover my fixed costs (hangarage, legal but minimal insurance) AND at least 200 hours of flying. But that's for a very humble 3-axis ultralight, and the hangarage so small that I must fold/unfold the wings after/before every day of flying.