Thanks for various advice.
Today, local "Indy" garage had a look, and the toe-in both front and rear were some way off the MG spec. He adjusted them, so now front is exactly 0 degrees, and rear is 0 degrees and 32 minutes - pretty well exactly as per MG spec.
Have yet to try out the car at speed, but hoping that change will have a positive effect.
He tells me camber and caster are not adjustable on standard spec MG TFs - but his print-out shows that the camber angles are different for each side (true of both front and rear). Should I be concerned, or is this pretty typical on TFs, does anyone know?