And don't run different tyres on the rear.........otherwise you'll f-ck the Quaife Diff.

So basically a 17 will be no good if you get a puncture in the rear, and a 17 won't clear the front brakes..........looks like your only option is spare 18, the same as is already fitted. Looking at the low profile tyres, I think it will still fit in the spare wheel well.

Could always take of a wheel and try it to see if it would fit.