Well, if yours was a non-S Cooper Clubman, it could have a 115/70R15 90M spacesaver spare.
That has part number 36101508367 and has a list price of £131. So ask the dealer's parts department to search for a non-S car.
If they really won't be that helpful:
a) hit them;
b) look for a non-S R56 hatch on the dealer's forecourt and give the registration number from that to the parts department guy, assuming he's still speaking to you after being hit.
A cheaper solution that I've read about is to use a secondhand spacesaver wheel off a Rover 45, but I don't know the ins and outs of that - you would be better off asking that question on somewhere like TotalMini where there are more members. I've just looked on eBay and there is a Rover 45 spacesaver there for £35 + £6 delivery. It's a 125/80R15 but that sounds a similar diameter but a little more width than a 115/70R15. I don't think the small difference in diameter can matter as the spacesaver wheel is far smaller than the other wheels on the car.