I noticed my contact quite quickly so no damage to body or tyre. All is fixed and ok now. There was a small area just inside the rear wing where the black under seal had been polished back to the paint. The contact would only happen very occasionally when fully loaded with 2 people, usually on a corner and with the suspension close to bottoming.
My car was not built absolutely symmetrical. There was more tyre to body clearance on one side than the other. I fitted a panhard rod in the first instance to reduce the body to axle movement. . Also I used the panhard’s adjustment to precisely centralise the axle within the body as things were a few millimetres out of symmetry.
I’ve also taken other measures to stop my suspension from bottoming but that was very specific to my car.
I do plan fitting the same tyres again as soon as they are 5 years old
