I tried a different SD card, 2gb and also not SDHC.

Split into two partitions and still the same issue. It seems it does it on 7.5.3 or above for some reason.

I've installed 7.1.1 instead and it seems happy.

Going to try another fresh install of 7.5.5, but just do things bit by bit so I can try to identify what might be causing the problems.