Just discovered this website - Priceline.co.uk
It's been around for years, can't believe I haven't used it before.
Essentially you select a region in a city, select a star rating for the hotel, then name a price your willing to pay.
There are some loopholes you can use and help from websites - I've used moneysavingsexpert and betterbidding.com for guidance.
So far I've saved 30-50% on the standard internet price of hotels. Wish I'd known about it before.