Managed to get sufficient in-focus to use the DS filter. Not sure I like the image any better even though I used 60s, 120s and 180s subs instead of just 30s.

The solution to get the extra focus for the DSLR and DS filter was not obvious and involved using 2 standard camera filter stepper rings, moving the primary mirror up the tube using the collimation screws and some jiggery-pokery.

Still have that horrible green/blue colour caused by the filter that I haven’t quite been able to white-balance out yet, and there is significant coma from the telescope.