I spent some time checking my alignment last night (firstly with EQalign and then a 10 min drift using K3CCDtools). I also tightened up a few things and ensured there were no cables dragging on the ground.
After all that I gave it another try (with the ED80 rather than the 10" newt so 1/3 the focal length) but did get a much better result with very little drift over a 2x4 minute subs. The next step is to try the 10" again.
Off to Bunnings to get some Velcro today.