Hi Dave
Sounds like the temp sensor on the flow. Mine did it last year. Think it cost about £12 in shop. I went to LPS supplies (adjacent to Ravenheat offices). They had a box of them on the counter. 10 min job to unscrew, bit of PTFE and screw back in. Hey Presto, can get in shower now without the fear of getting frozen!
This is the bit http://lpssupplies.com/csihe-sensor-0007ter050050-p-448.html
Tony T, February 2010