Thanks everybody for replies. Will probably not bother with extension. FYI alloys were already replaced after one year. Thanks to this forum I pointed out a few noticeable "bubbles" at initial service and
wheels were replaced without further ado with, I think, refurbished ones (which have a sort of pearly finish and to my mind look better than originals!) The replacements appear to be doing pretty well - even though I live only a few yards from the sea, but I do at least wash the wheels regularly!