Spare Wheel diameter

Hi,
Recently purchased 2004 MGTF with 16” eleven spoke alloys, which were ordered and supplied from factory.
However the  unused spare wheel is 14” as shown on the original MGR Supplier order specification.
My question is ……Did MGR fit 14” as a standard spare wheel, irrespective of what wheels were fitted to the car?
I assume the 14” spare can only be fitted to rear axle, as will not accommodate 4 pot front brake callipers. In the event of a wheel change, is  this safe to drive with 1x14” wheel and 3x16” even at reduced speed?
Many thanks 

14 inch steel spares were generally only supplied on models which had the standard brakes and 15" alloys.  

16 inch wheels were standard on the 160 and on anything with the 4 pot AP racing calipers as required for clearance. These would all have been supplied with a 16" alloy spare (usually painted black) with an omni directional tyre that could be used on any corner. 
Although 4 pot calipers could only be specified paired with 16" wheels its likely some may have ordered 16 inch wheels as an option but with the standard brakes.  However if yours has 4 pot brakes it really should have a 16" spare as a 14" is only going to be of any use on the back.  So yours wont have left the factory like that: either someone has later uprated the front brakes or the16" spare was lost and someone has just thrown a cheap 14" in there not understanding the limitation

There is a calculator for comparing the rolling circumference of wheel/tyre combinations here   you just need to put in the wheel diameter, profile and tyre width of each and it works out how close the two are. 

If you go to somewhere such as 'Bitz for a good used 16" wheel and tyre it's best to tell them it's intended for use as a spare and it needs to be able to clear 4 pot calipers.

Many thanks for reply, the car was ordered and supplied with 16” alloys and 4 pots, and what you say makes perfect sense, as I reckon the spare has been switched at some point, the search is now on for the correct replacement.