1.9td AAZ engine running awful.

[dub warrior3]

For the last wek or so my engine has started to run really badly ever since it went in for an M.O.T.
It went in running fine. Obviously IT WAS NOISY, but it still pulled really well and felt quite strong. The guys at the M.O.T station said that they had to put a bit of oil in the car as they needed to rev it a fair bit for the test (?).
The car now feels quite sluggish and sounds really tappety, but its no longer running hot. It also struggles to tick over without the choke thing pulled out.
Could this be a valve problem or a turbo or head?
Afraid I know very little about the dreaded diesels.