Thanks Mother, I tried the new install (mvtools2.dll) and AVISynth then complains that it can't find the function MVAnalyse, which appears to be now MAnalyse (?) in which case the usage also seems to have changed.
For some reason I was using a really old version, and it looks like once I've upgraded to the last of the 1.x branch it might have fixed the problem.
And I've finally got it working and then I get an Assertion Error and AVISynth explodes...
Might give up on this one...