Fitting the dowels to board number 11 for connection to board 5 has worked much better. Firstly, I drilled the recesses to 28mm rather than 26, meaning I had more wriggle room in the alignment on board 11. I used more quick set epoxy this time, so that edges were filled as well as back surface. This seems to have gripped the dowel much better. I used a thin piece of paper (an old Tescos receipt) between the connection to prevent them gluing together while the two boards were clamped together upside down on a flat kitchen floor. Letting them set in this way has meant the top surfaces are about as level with each other as they could be. Board 11 is an old 3 foot outer radius curve from a build for a test track I did long ago. It's top surface at that end wasn't entirely flat, so that will need sanding down to bring it level with board 5's top surface. It needs sanding down anyway as it has detritus on it from the previous test track usage.

Some pics below show the finished position, and how the intermediate expansion board 9 has to be able to fit reliably between 11 and 5. I've yet to do the dowels on 9's 'B' end for connection to 11's 'A' end. The trickiest part of the whole fitting is making sure the pilot holes for the dowel screws are exactly dead centre in the dowel holes. If they aren't exactly centred, they stand a chance of pulling the dowel part out of position, glue or no glue.

Now that I know my retro fitting 'system' works, I feel more confident of applying it to the rest of the boards, new and old