I picked up a super cream/black stripy rug from Home Bargains for just £10, and painted the backboard black - and it's made so much difference. The whole space is really calm and restful. There are still a few bits of turquoise about from the time my old summerhouse was chocolate and turquoise - but I am not chucking out good throws and cushions for the sake of it!
I am doing a little bit more to the ceiling every weekend - it's going to be the same white with black beams - and I think I will replace the papered panels on the doors for something blacker. I think I may have to make a cover for the knitting machine in black if I can get something from the charity shop - not sure I can live with that purple - it stands right out!