I've got a 1.8TDCI MK4 that I've had issues with since I got it a month ago. The leak-off pipes were replaced, and it still wasn't starting right (taking ages, plume of white smoke etc). The garage tells me there's an injector issue after I get it back from having the leak-off pipes done, because I called up and said about it not starting and running well, still. I pay a £500 deposit for 2 injectors, and I was told it "Might be more" at about £1800 in total. So I get it picked up by recovery and taken to the garage. 

The mechanic comes out to greet me upon pickup and tells me it came up with a fault code. Only, the mechanic who picked the car up from the recovery company checked it the day before and found no fault codes. They one injector had gunk all over it it, and another wasn't working properly. I understand they change them in pairs so I guess that makes sense but something doesn't sit right. Anyway, take it home...5 hours later I come to start the car (waited until it was cold outside to check) and sure enough it starts and seems to be some form of misfire and delay when driving now. 

The total bill for 4 injectors was actually £1400, with a new sump (as it was pretty dented), fuel filter / oil and filter change. 

So my questions are, any idea why it's not running right? And does any of this sound dodgy? Thanks!
 

It's quite common to change all the injectors at once on older vehicles rather than one by one as if one of them is faulty it can have an affect on the others. 

If it's struggling to start in cold whether and then runs fine when it's warm I'd suspect it's more to do with the glow plugs seeing as these heat the cylinders up when it's cold. 

White smoke = unburnt diesel, either overfuelling due to faulty injector(s) or engine has low compression....in 1 or more cylinders. As you've eliminated the former (at some cost!) maybe get a compression test done.