modeller, I've had exactly the same experience. B&Q quoted me �4300 to do my bathroom last year. this year with 50% of the bathroom goods and 50% of installation it came to �5300.
I then found out that they artificially bumped up their prices pre-Christmas to make this "offer".
apparently they were selling a toilet for �50. pre-Christmas it was put up to �120, so now you can get it half price, �60!
trading standards and the office of fair trading aren't interested. they can basically charge what they like, so long as the goods/service actually were the pre-sale prices they state.