Just a quick update; following advice on here, I ditched the standard Thomson 'free' car, and opted for a car from Alamo.
I checked prices over the course of a few weeks before booking when I saw the price gradually rise;
costs from 9/1/17
Economy: Mitsubishi Mirage or similar £316.48
Compact: NissanVersa or similar £338.49
Midsize: Hyundai Elantra or similar £355.40
Economy: Mitsubishi Mirage or similar £322.48
I checked after I booked and gradually the cost was still rising, so I was quite pleased.
On the off chance, I checked again today, only to see the same car had gone down to £264!
I logged in to my booking, cancelled it, with no charges, did a new search, and suddenly the price had gone up. obviously due to cookies, so I did the same search on another device, and got the cheaper costs again :)
Ive now rebooked but gone for Hyundai Elantra or similar for £279.45! so that's 2 upgrades higher(if you can call it that!) and still a saving of over £40.
Just a heads up for anyone else out there, prices seem to have fallen!