This evening has been measuring and planning.
Longest requirement is going to be the GWR HST. I've got the 8 coaches, which measure 97" (sorry for the mix of imperial/metric coming along but I like to complicate things). The HST power cars are 235mm so about 9 and a bit inches, or 116" in total.
There is nowhere I can accommodate that length of train (longest current siding is about 12" too short). Planning the roundy roundy bit, 'if' I use one of the pizza ovens in the other corner, the longest run I have is 130" from the curve [i must do some drawings/photos of what I'm taking about to help].
So 130" - 116" leaves 14", which should be enough for a 8.5" point, plus 4.5" clearance after the point - not sure that is enough?
I think it's all coming together in my mind at least. I'll post some plans, photos of the areas to be developed when I get a chance.
Phil