Thanks Petra
OK - first light has occurred...
I've tweaked collimation so that I get a concentric disk at centre. It's amazing how this is lost as soon as you start going off-axis. The concentricity is quite critical.
And.... diffraction spikes on bright stars. Weird!!!
For a refractor guy, this is very strange. I'll let it get darker outside and see if I can see clusters as dots or cross-hatch fields...