I finally organised a visit by our local OSMG (very lovely chap) and he took a look at the 401g it was skipping stitches and generally 'not quite right' .
I thought it was a timing issue related to when I did some work freeing the cam followers. Nope... the needle was in the wrong way round...
Doh!
The stitch selector was stiff, needed the screws on the knob tightening rather than additional oiling or work
Also I hadn't noticed the upper tension was weird... he did a load of work adjusting the tension and also found that it was just one of those machines that need a firm tug upwards to get the thread into the tension disks!
So it's now officially back up and running...
I just have to stop berating myself for not spotting the needle was in backwards