http://www.motuk.co.uk/manual_410.htm
as long as the sidewall height (aspect ratio in this instance) is approximately the same and results in an identical rolling circumference, it will pass an MOT. for example my twingo comes from factory with 185/55/15's but the spare is 175/65/14. this results in an 0.34% difference in diameter of 2mm. the guidelines are for no more than a +2.5 % difference when using a spare which is sectioned under the constructions and use regulations for motor vehicles
if an MOT tester is going to fail it without calculating it then he doesn't know much about tyres and shouldn't be testing in the first place. it's just simple maths lol