I suspect the new tyre co-incidence is a red-herring (the directional arrows are pointing correctly on the sidewalls) and think my problem has been compounded by:

1/ Something worn (a bush and bearing, however slight it might be, because of the extra leverage created by 8cm hub extensions at the rear and lowering)
2/ Coilover springs at their lowest setting, they are stretched and more springy than they should be with say a mid setting
3/ Coilover dampers not capable of adjustment, the setting should be much stiffer, and even stiffer springs if possible, as the wheels are far more outboard than stock, again more leverage

If you Paul or anyone else knows where I might be able to acquire adjustment blocks or spacers for the tie bars, that would be great.