Have decided to "bite the bullet" and buy new 20" rims and non runflat Toyo Proxes Tyres. Far better (IMHO) as an overall package for a variety of reasons than sticking with the one choice in 21" tyres at such a high price per tyre!!
New wheels and tyres will be fitted mid week and I will just store the 21's till I sell the car (or maybe sell them sooner).
Did try the Irish company referred to in this thread, but nobody returned my call.