It transpires the 145 80 18 I bought is in fact not suited for use on an X6 after all, the optimum tyre size being 155 90 18 or 155 80 19 if on an alloy temporary rim.
The question is with a diameter difference of 39mm, I imagine even for temporary use the uber sensitive BMW transmission may complain?
Changing just the tyre is over £100 fitted and simply not cost effective, so it looks like I'll need to buy another complete wheel

What a bu££er.