Thanks to all on here and the Templot Club for advice, some of which was ignored but hey, it's all good.

The solution is somewhat of a compromise as is often the case and I have gone all round the houses to arrive at the obvious.

here's the current plan:

I have gone with a GWR 9ft heel #6 turnout which gives 10" of total track length from switch tips to the 11 ft marker, take an inch off for a half buffer and I will still have enough room for the largest "home" loco on the layout which will be a 57xx tank. Keen eyes will notice that I have extended the layout by a foot to the right, but this is for the future should I decide to accommodate larger locos.

I think I got a bit distracted by trying to make the release as long as possible with the existing baseboards for locos I might never own (class 3 tank etc) and compromising the track design in the process. Glad I have seen sense and replaced the #4 with a #6 turnout.

I will make the scenic area easily extendable so that any extension will be fully scenic - maybe a church spire covering the backscene join?