Thank you all for your replies.
We've had the car for a week now and had chance to run through a tank of petrol. Doing a litres/ mileage calc. it works out to about 38 mpg. Looking at the specs it's about what it should be, so I hope there isn't any major problems with the engine.
I could check for voltage across the 'eng management' LED with a meter but that would entail removing the dash pods glass, pointers and face again!! (I've only just got the pointers on in the correct positions from last time)
I still don't really know if the 'eng management' is faulty?
I think I'll run with it for a while, my logic being that it can't be too bad if it's giving 38mpg and starts well hot or cold.
When I get time I'll run it to a local garage to have any 'codes' read bearing in mind that I have probably added a few myself playing with sensors etc.
Thank you all again for your help and I'll post when I get any info worth posting