My Previa is running very hot although the temp gauge stays in the middle and it never over heats, but the driver and passenger seats get very hot!!!
I just can't work out what this could be
Can any one help me please

It could actually be your automatic transmission. Too much or too little oil in it will cause it to overheat and heat up the entire mid section of the car. With an extreme sense of urgency check the oil level. YOU MUST do this in accordance with the owners manual which is.

1. The car must be at operating temp so after at least a 10 mile drive
2. The engine MUST be idling
3. Before checking the oil move the gear shift from park through all the gears, pausing briefly at each gear and then back to park
4. Open the inspection hatch under the passengers seat (as you are in the UK)
5. Withdraw the auto dipstick, it is the red one to your right.
6. Wipe it with a cloth and then reinsert and then withdraw again, The oil level should be between the HOT H-L marks.

The oil should be a red colour and should never ever ever be above or below these two marks. If it deviates from these specs have it sorted ASAP or you may soon find yourself walking.