We always use holidayextras.co.uk for a hotel + parking deal - for the past few years the best deal has always been at the Park Inn on the Bath Road - the deal includes 15 nights in the T3 long term car park, so you keep your keys and get the shuttle bus the short distance to the airport. The total bill hasn't been much more than parking on its own.
The Park Inn is nothing special; it is huge, and very busy with coach parties and air crew, but it is clean and fairly quiet in the rooms. It is also only a reasonable walk to The Pheasant for our pre-holiday meal.

They also do family and triple rooms.
If you do stay in one of the Bath Road hotels, don't forget all of the service buses to and from the airport are free. I wouldn't use the rip off hotel hoppa unless I have a mountain of luggage and/or a tribe of little'uns in tow.
Nick