The mechanic who wanted to charge you £550 to "rebuild sensors" - Never ever go to him again. He's a conman and a thief.

Secondly buy yourself a fault code reader such as this one. for less than £15 it will more than pay for itself the second you plug it into your car for the first time.

Armed with that you need not bother taking it to a garage for a diagnosis - as all they do is plug the car into their machine, and charge you handsomely for the privilege. If the engine management light (the glow pug one) has come on it will have stored a fault code - get the fault code and then Google it. If it's P0251 and P1211 sell the car on (seriously). If it's not, it will probably relate to the EGR, possibly the fuel pressure regulator, maybe the inlet metering valve - but it will hopefully help you pinpoint the problem area.