Looks like it was indeed a calibration problem. Bunged it up at low altitude in the back garden to check the fault was still there - it was. Landed, switched off, went through Stick Calibration - ye gods what a palaver, never done this before, had about 6 attempts before I got it done before it timed out! Anyway there was a small tweak requested to the left stick. Took off again, and this time the heading was rock steady.
So thanks folks, a problem solved, for me at any rate. 
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