I took another look at the MGF over Christmas and realised that the expansion tank was full to the top, and this was when cold. This is my fault, last time I topped it up I stupidly overfilled it. So I drained some off, down to the correct level. The overheating problem appears to have gone away. I went on a 20 mile run to test it and the gauge stays rock steady at the right temp, whether I am moving forward or standing idling. Could this have been the problem?

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