I agree that the Novotel is overpriced. For tomorrow night, there's a Best Western for about GBP60, or the 'Vismaya' for GBP28. Both are within a few miles of the airport, seem well regarded on tripadvisor and possibly have airport shuttles. There are even cheaper hotels in the area also...

I spent 7 hours (daytime) in BKK airport recently and it wasn't pleasant... you have 15!