The point I was trying to make, was that an instructor should be paid 'quatum meruit' (as much as the job is worth). In other words to be paid for all tasks related to the job, including briefing, fueling, administration, pre-flight checks, de-icing and flight instruction. Not just for flight instruction.
The best solution is for the instructor to be paid an hourly rate for the time he/she spends on duty. The hourly rate should reflect the non flying duties, so if the school does not have admin support or a refueller that is replected in the pay. This is easier from an invoicing and payment perpective.
My experience is that students are prepared to pay for a four hour block training session (morning/afternoon/evening) at a fixed hourly rate, regardless of if we are airbourne.
My understanding is that at some schools in the States charge the student from when he/she walks through the door. Perhaps someone from over the pond can clarify this?