I had a day off today and managed to grab a couple of hours to get the two boards back together again. Before I could join them together I had to modify one set of legs so that they were exactly the same height as the other. Having made them in two pairs I made the cross piece a slightly different way so they were a few mm out; now they are identical both boards are completely level.
My rebuilt end to the Three Bridges board fitted very well with the Underhill board... once I had removed one piece of bracing I added during the last bodging session before the Port exhibition.
There are a few trackwork issues on the new board that need sorting out but fundamentally it all worked once I had cleaned the paint of the track and cut back a bit of over enthusiastic new greenery. A couple of the points are bunged up with paint so don't throw properly with their motors so I'll need to get them dug out. The baseboard join is OK but there is a slight height jump which can be adjusted with a bit of fiddling.
Once the track is working properly the only real things to get sorted are some drapes to go under the baseboards to hide the legs etc and something to fill in the corner between the boards where the lighting rig plugs in. I have an A4 picture frame with a map of the island and an explanation but it needs some tidying up somehow; I have a few ideas.