Thanks for that
I have an update however, they recommended getting the sensor replaced as this has resolved problems before, and as it only take roughly an hour, at a cost of £275 I decided to go ahead as they can start straight away....
After 1.5 hours including a test drive, I was told it was done, although the sensor they thought they had in stock was not the correct type, but they noticed that the solenoid was very dirty, they didn't have any of these in stock either, but they managed to clean it up 'significantly' (they also did an oil and oil filter change), they were very optimistic and this came cheaper to at £177...........
and so far so good, it's had 3 starts (4 inc test drive) and no sign of warning light or power loss, so grabbing onto wood tightly, I believe it's problem solved, time will tell.
I was getting was intermittent warning light, and power loss at different levels (sometime feels like 50BHP down, sometimes 100bhp down) and sometimes I didn't experience any power los at alls, but would sometimes kick in half way through a journey.
If you get these symptoms, might save you a few ££ by going down this avenue