Hi Nicola, today we had the same problem and this is what I found on Gainsborough's site. Hopefully this helps. But we are looking at a new shower.
if you go on gainsboroughs site they have a diagram there and you can actually order spare parts. Yours sounds like the outlet pipe and cost nearly a tenner. but advise is that when it leaks behind the unit do not use!!!!!!
kind regards Marion
1. Outlet Pipe
2. Shower Engine
3. Front Cover
4. Badge
5. Control Knob
6. Rail End Caps
7. Riser Rail
8. Hose
9. Hose Ring
10. Handset Holder
11. Handset
Marion
December 2008