I had a further mess about today, if I now set the wheels to about 0 degrees toe it does still self centre but not as well as with a degree or two of toe out. Dialing in a bit of toe out makes a big difference. I also experimented with tyre pressures but got less marked results from that. To my mind it now complies with the IVA rule with it set so that handling should be Ok so I am a happy camper.
Thanks to all those that replied.
Enoch