Thanks for the input so far guys. A new coolant temp sensor arrived today, but unfortunately hasn't fixed it. I had my fingers crossed and everything, and for a short time (minute or so) it seemed to run smoother, but I took it out on a test run and it showed the same symptoms again. It seems like it becomes more unstable when it's warmed up a bit too. i had a couple of runs of normal acceleration, but for the most part it was the same, lacking power in the lower rev ranges when my foot was to the floor, and only picking up later on. One new thing today, which I hadn't noticed before, was that when I held it at 1250 revs (in neutral) it was fluctuating up and down about 100 revs either side. I didn't test this last time, but at this point it wasn't misfiring (during the minute or so of it running smooth today).
The crank sensor is another thing we had thought about, and were hoping it would have showed up on the diagnostics, but nothing did. The mechanic didn't use Vagcom though, so I don't know how reliably the system he has performs on older engines. I'll have a look at the vac lines, but I'm pretty sure they were checked.
