engine management light on?

Hi, if any 1 can please advise me why has the engine management light come on? I have noticed that it is jigging when being driven, especially in 2nd gear. As you carry on into 3rd gear an so on it stops jigging(shaking like). The light is on at all times. I have took it to local mechanic whos' advised to have a diagnostic check done. I paid £35 for this and it asked to change the coil and plugs, at a cost. But the problem hasn't been fixed, car is still as it is, but ive paid nearly £110 and got no where.
Could anyone please let me know what I have to do next? I av bin to mechanic again, to try to get that chap to av a look, but after a wk, he hasn't turned up. I use me car daily and it is so annoying i'm not getting anywhere. Thank You Any help much appreciated