I finally got around to taking 150 over to Himself's place yesterday to check if the alterations I made to the bogie pivot bolts did the trick - in other words that I runs through Rhoslyn Cutting without scraping the sides.
The good news is that there is now clear daylight along the right hand side. Phew!
We took it for a test run all around the layout.
The only issue we identified is that there's still a slight issue with the bogies having enough clearance around the tightest curve - Dragon - where the spiral exits into the fiddle yard.
Himself will take a look and make some final alterations to fix that before it goes to be painted.
That won't be far away - it's really just the underframe which needs to be finished now.