Thanks for replies - I'm not going back - the light came on so I went to my local place as I didn't want to drive too far with the light on. They told me the sensors had failed (both) - they showed me the one's they took out - they were bendy and in poor condition so I assume they were faulty. When I got the car back the light was off - it had been reset but it came back on when I next started the car - they had a look and said the fault was the heating circuit - probably the wiring. They spent all morning on it - no charge but when I went to pick the car up they said they had turned off the EML - not sure how they did it - I assumed it was the bulb. Answers to questions:
1. They are aftermarket sensors.
2. Which fuse should I check?
3.I don't have the fault code -will get this from them tomorrow - hopefully when they renable the EML.
4. Yes it's a ST170 - 2003 model, 73,000 miles.

Hope this helps - will post updates after I've been back tomorrow!