I think the next step has to be sorting out the spider and secondary mirror assembly.
Now I’ve had some time to consider options, what I’d really like to do is to build a secondary assembly similar to those used on more modern newtonian models, using two pieces of round aluminium bar (one cut at 45 degrees for the mirror mounting). TS sell such items though they’re far from cheap so I’ll have a go at making one myself. I can then mount the plate that I already have on the end and fit the mirror to that.