I would think that £100 per week is not excessive. Fuel prices have gone up a lot over the past year, and the prices would need to be passed on. However, if you booked your holiday and agreed a price for pool heat, I would have thought that the owner would feel honour bound to stand by that agreement. Perhaps you should get in touch and see if you can come to some arrangement about the increase.

Have agreat holiday, and don't be tempted to cancel the pool heat!!, you'll need it in December