There is multiple ways of paying FI's
Straight flight hours.
Retainer and flight hours.
Different rates for different things ie Trial flights, PPL, Night ratings, IMC
You can be on the books on PAYE or self employed
Part time they tend just to invoice.
I was on 400 retainer and 10 and hour flying about 900 a year PAYE which worked out about 14k back in 2003 full time.
The part time guys were on 15 quid an hour invoiced no retainer.
With a dual rate on a tommy including landings of 97 pounds.
I don't have a clue what the current rates are.