maybe youve got a hole in the exhaust... if youve got a hole itll never pass, talking from experience here. dependant on how fussy the MOT people are have a listen under youre car when you take it back, or ask to have a listen cos youre menat to only get 1 free retest (although some give you a couple) before sunday, listening for a blowing sound. if you find a hole just cover it up with some exhaust repair or aluminium tape at worst... mine had a hole and i just wrapped that section of pipe in aluminium tape and it passed
... 3rd time lucky
also if theres no hole some engine de-coker or whatever it is (redex style) might give a bit better reading... is it far out?