I tried out the centreboard in the centreboard box yesterday (sensible place to try it out) and it didn't fit... very well. It was VERY tight.
Even though there should be plenty (well, a couple of mm's) space I think I know what went wrong. I used a large concrete block to 'clamp' the centreboard box together when I was gluing it. I remember that the brick didn't go all the way across, meaning that it left a slight bow in one side of the box. So, the 2-3 mm's of free space has disappeared and it is now too tight. I should have had a 25mm spacer in the centreboard box to keep it at the correct width.
I'll jam a thick spacer in there for a couple of days and see if it takes the curve out of it and gives me the correct fit. Fingers crossed.