I am unsure of the rim width on the S1, it would be interesting to find out.
I have a feeling they are 8.0" wide, as on 225 tyres the rim does not protrude.
My thoughts were if all the later vehicles had wider rear tyres it might have been done during the Morgan "development" cycle to improve the handling and make it more neutral - hence my interest. (Plus wide tyres always look good!)