James, thanks for your reply.
Well it doesn't seem to matter what I wear but I am now wearing a waterproof jacket to try and eliminate the seat as the root of the issue.
I can't prove that the seat is the problem but there is another report that an Audi has caused some skin issues although this is far more extreme.
http://www.msn.com/en-gb/cars/news/man-is-allergic-to-his-new-car-and-can-only-drive-it-for-40-minutes-a-week/ar-BBpbHks
It seems odd that the irritation is only on my back and not on my legs. My car had a lifeshine treatment and I am concerned that this may have caused the problem.
I have now thoroughly shampooed the seat and hopefully removed any potential irritant.
I suspect I will never find out what has caused the problem but the coincidence does seem a but strange.
Just one of those things I suppose.
John