Again, thank you for the replies. I think that I need to explain why I am so concerned about this subject, as in reality, I am quite a balanced person, and after reading over the posts, I might appear to some as being a right plonker :-)

Every summer we, as a family, drive from UK through several European countries to Ukraine for our summer holidays. It is a 4200 mile round trip, plus whatever mileage we clock up while we are there, so rather than leaving it to the last minute, I am looking at getting things ready to remove any last minute decision making..... and July is only 3 months away.

So, after reading the replies, I now understand that run flats aren't mandatory to insurance companies, if having to make a claim, they just worry about the tyres being legal. In which case, buying normal tyres plus a spare and carrying the associated tools is worth considering.... and are much cheaper.

It isn't the diameter of the tyres that I was talking about in my previous post, although I know that it is very important that they are the same, if the tyres were different sizes the car would suffer from wind up, I was meaning the tyre width.

Due to my car having wider tyres on the rear than the front, I was wondering if getting a spare the same dimensions as the front tyres would be a better idea, as I assume that a rear tyre fitted to the front of the car would surely rub when steering. As much as the thin "get you home" tyres are useful, I think a full sized tyre in the boot, that if needed, would allow us to carry on our journey at a reasonable rate, plus I have never liked the idea of driving a car with a bicycle wheel on one corner and feel that a larger tyre would be more beneficial considering the journey that we will be making.

As for tools, I already have a 2 tonne bottle jack that will fit into the under floor storage space (I have tried it for size already) and I wouldn't set off on a long journey without carrying a certain selection of tools anyway.... of course boot space is going to be a concern because of there being a lack of other stowage in the X5 other than the two small cupboard places either side of the boot, and a spare in the space where it would normally be kept.