The Travelodge option... yes, the LHR central one may say it is closer, but by the time you have skirted around the airport, or got the train transfer, you may as well just go to the T5 one...
I think the Thistle is quite reasonable, not the most amazing hotel in the world, if not some what in need of one heck of a make over, but if value is what you are looking for, worth a look.