Hi Peter,
Firstly, thank you for responding to my query.
I had measured the wheel well on my XF and it seems to have enough space for the 245/45 R18 tyres to match those on the car. Am I missing something? In the 18 months I have owned the car those big , deep tread tyres seem to accept all the potholes thrown at them! Because of my trust in the tyres dependability it is likely (having read Martin's notes about the travails of using the provided sealant) that I will settle for a basic 18" space-saver wheel just in case. I am covered by my insurer for breakdowns (the cars, not mine!) and my plan would be to call them out to change the wheel, simply because there are few safe and stable places to do the job on the roads.
Martin, I am unsure if this reply will get to you as well, but if it does thank you for your swift reply. I will get my local garage to give me a budget quote for all the recommended items to be renewed at the same time as the cam set. Will post back to you with the result.
Regards
Geoff