Maybe a daft question, but anybody know why 50 mph speed limit on spare wheel?  It is not a space saver but slightly narrower.

The Construction and Use regulations state that if the two tyres/wheels on the same 'axle' do not match then the car is limited to 80kph and can only be driven for the shortest possible time until the correct wheel/tyre is re-fitted (i.e., only in case of a tyre/wheel problem with the original fitment).  And the speed limit is entirely reasonable.  In fact my opinion a non-matching wheel/tyre size should not be allowed under any circumstances.