The filter was also on loan from Stephen.
I have a DGM Optics NPB filter permanently mounted to the scope on a filter slide. I believe it has a similar absorption profile to the UHC, and would have been equally suitable. The problem was that the nice Zeiss required a lot of focuser in-travel, & I couldn't get the eyepiece to focus with the slide in.
Many thanks for your information Les. You've given us more stuff to try next time.
Phil