Well just got the car all sorted, for the MOT today, but on the way home the Engine light came on

GREAT!!!!
Little history about the car, i got it a week ago, as the car had been sitting for 6 months and not moved,
I got it for a good price with the view to sell it on, but kinda like it now lol, so got it all running, normal stuff battery dead, sun roof dead, all now done, and the running, when i drove it home for the first time it was a dog, but that im sure was to be expected being sat for 6 months in all that cold weather lol.
So cleaned the inlets out,TOTALLY GUNKED UP and repaired a water leak, and now runs a little better, at that point no engine light
The revs at that time were sitting around 650 at idle ( i thought a little high)
Also sometimes the revs would hold at 2000 pm, drop then rise again for a few mins, then it would settle down.
But now the revs sit at 500 RPM at idle but now the engine light is on,,,,, any ideas
its not reving as much now, and also when i give it some stick it splutters at the higher revs, thinking it prob just needs a dam good run or a service,,,,, would this clear the light issue or do i have something more serious?

also is it the pedal trick or paper clip test i need to do

if so does anyone have a link on how to do it .
thanks in advance
tony