My first thought would be ditch the oil & put in some decent stuff like Millers
http://www.millersoils.co.uk/whichoil.asp
Change the filter too.
Normally, oil is burnt because of worn bores, valve guides or turbo (or a bit of all three), but if it starts easily enough, I would also blame cheap oil.
Not all garages put in good oil and it gets burned by the combustion process.
It's a cheap-ish fix at around 40/50 quid, but a lot simpler than removing injectors.
Next, fill the tank with premium diesel such as BP or Shell's best (it works out a couple of quid more than the cheap stuff and is FAR better than any supermarket shise
Once you filled the tank, add some redex and take her for a high speed motorway run after having booked it in for a retest. If stopped, you tell the plod you are on your way for a pre-booked mot and were advised to run it up to clean the exhaust & injectors in order to pass emissions test.
Don't go over the speed limit, just stay in 4th instead of 5th for around a half mile or so, then drop into 3rd as if overtaking a couple of times.
Then report back