Had it again today. This time I turned right after T/O. The error was less - 90 degrees. I'm sure with a left turnout it would have been 180. I took the time to scan the HSI during the ground acceleration. It rotated 30 degrees on the ground despite a backtrack and two mins at the threshold to let everything stabilise. By 900' it was out 90 degrees.
The worst of it was that at no point did the heading flag appear - well not untill I unslaved and started to realign, then it came up.
Its just a theory, but I'm sure its environmental. The kit itself has been given a clean bill of health. At first I thought of iron deposits but the geology is wrong. New phone masts went up 6 months ago. I'm told that the Emb 145 (not my aircraft type) gets 'comparator' warning captions (10+ degrees difference between HSI/compass)accelerating over buried power cables at a big regional airport. There are no such features effecting my T/O.
I will explore it further (if VFR) in a few days when I go back there, I will also speak again to the ASR team.
Any gen on electromagnetic interference appreciated, or other pet theories.