It seems Stockport Decathlon are selling of their remaining windsurfing kit at half price. I was in this evening and couldn't find anything I needed, but there did seem to be:

A selection of alloy booms, which at half price will be about £27
Two large (180-240, I think) carbon booms which should be £95 each
A fair few masts. Most looked to be reasonable carbon content. All were standard diameter, no skinnies.
A few other bits and bobs

Normally I'd say support your local shop, but I got the impression this could be the end of their windsurfing section, so if you're local it might be worth popping in to see if there's a bargain there for you.