Ford Focus 1.8 TDCi bad idling

[richard humphrys]

Hi everyone one I brought a Ford Focus 1.8 TDCi back last year it's a Mark 2 facelift my problem is that when the car gets warm it starts to Idle very badly at first when I bought the car I didn't really notice the problem until I was halfway home as I spoke to the person I bought it off he said there wasn't a problem and that it had a new injector in number 3 so a couple of months went by I noticed it starts to get a little worser so I brought it into the garage unfortunately they couldn't find anything wrong with the car he did say it probably needs new injectors so I went and renewed a couple more but I don't really want to spend anymore money on injectors if it doesn't cure the problem I have checked  the EGR valve that is very clean the person did say it had a new clutch when I bought the car when it starts to Idle bad I have noticed that if I put the front windscreen heater on and the back windscreen heater on it does calm down for a little while I don't know if it can be a crankshaft sensor which is faulty or it to be an electrical problem I really don't know where to look now any information would be great thanks