Just bought this 07 Cooper S which comes with 18" wheels and 205 40 18 tyres.I cant find these sizes for winter tyres so am looking at 215 40 18 winters.Has anyone else done the same or any other tyre size that would fit ?

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i've got the same wheels,i couldn't justify the expense of getting cold weather tyres fitted..

Im up in the North East had to abandon the last Mini a couple of times because of the snow.Only need 2 on the front and part worn would be enough so hope to keep costs to a minimum.

Only need 2 on the front and part worn would be enough so hope to keep costs to a minimum.
Putting winter tyres on the front only is fine.... as long as you never want to use the brakes on a bend or even brake hard in a straight line. With more grip on the fronts than the backs, the backs will reach the limit first and you've got mammoth oversteer. 99% of the time it'll be fine, but the other 1% you spin uncontrollably.

i couldn't justify the expense of getting cold weather tyres fitted..

Only need 2 on the front and part worn would be enough so hope to keep costs to a minimum.
The cost of repairing body damage from an accident caused by driving with improper tires in the snow is likely to exceed the cost of buying proper tires ;)

Ive seen this argument time and time again.Im driving 200 yds on a snow covered road from my house to the main road which is clear.We ended up parking on the main road and used the mini throughout last years winter on summer tyres without incident just like we have done for the past 20 odd years with various cars.We only have a build of snow on the roads around our house which are never cleared or gritted now.

I wouldn't use 18" tyres in the winter, especially not 215/40 (wider than your summer tyres). Some go for 16", I use 17" just fine, but I wouldn't dare to have 215/40.
If anything, I wouldn't want those lovely rims suffer from salt. I have them too and I rather use 17" web spokes for winter than these babies :-D
Also, there are often potholes in the roads, that aren't repaired in winter, and you can easily damage the tyre and rim... It happened to me last year, RF tyred had to be replaced and rim fixed.