When I checked the cut part in the middle, I thought "that's odd, its 2-3mm too wide". Then I checked both ends, and they were spot on. So a bit of further investigation showed that the trusty sawboard had a very slight bend along its length! Not sure if its always been there, or for some reason it had warped.
Anyway, to fix it I clamped it to the factory edge of the MDF sheet, and used a router with a fence to trim the face part of the board parallel to the edge of the MDF sheet - vutting just deep enough to take the face edge flush to the top of the board. Then I re-cut the board edge with the circular saw to make it match the now slightly shifted (and straight!) fence on the board. So it works ok again now...
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