blocked 15mm shower pipe

Our mixer shower is fed from the hot water pipe feeding the baths hot water tap. Water just drips out of the shower but is ok at the bath. I therefore assume that a blockage has occured in the pipe from the bath tap to the shower. I've taken the mixer to bits and the water is almost non existant on the hot water side. Now as all the pipes are inside the platerboard wall is there any way I can remove the blockage without having to cut the wall out?
Alfie

What type of system do you have i.e a combi or a open vented system?.
How long have you had this problem and have you had any work recently carried out on your system?

Ok it's a combi but thats been checked however the mixer shower (Triton) was only fitted a couple of months ago and has never really worked.