Hi Cathy,
My T80si started running cold, so rather than get new bits and pieces I bought another shower (off ebay - silly me), anyway I had the new one fitted and it leaked from the same place - the stabiliser valve assembly. The part was removed from the older shower and fitted into the new one, and its absolutely fine.
So the person who answered and said "knackered" is so very wrong.
You can buy this part online for about £27. You can also purchase solenoid and thermostat and pretty much all the bits.
Having watched what went on and what went were, I'm pretty much now confident to tackle anything else that may in the future go wrong with my shower.
Annette, December 2008