Having both boards for Penlowry now erected, it gave me a chance to check my design against the real thing. Obviously I have no desire to rip up any more of the existing track than I need to if I can help it.
Looking at the design on my computer and then at the board, I had a Blackadder “Hang on!” moment* as I realised the three way point at the RH end of the loop wasn’t in the right place on my diagram. I’ve therefore put it back where it should be which had made the track geometry so much better.
I’m also investigating whether I can get the layout printed out on a plotter so as to avoid improving Sellotape’s shares.
* if you’re not sure what that means, try here, from the 9 minute mark. https://youtu.be/MYYEbR9xQmU