my s2 has just failed mot coz my abs light is on,the tentioner for the belt is at its tightest point but the belt doesnt seam very tight? could it hav streached.

 

if ya not bothered about having the abs working put a zip ty around the sensor and the light will go off

 

Could quite easily have stretched. Personally I'd replace the belt & have everything working properly. If you just ty-wrap the sensor, they may check this when you take it back for mot & fail it anyway....then not only will they go over the car again as you've tried to deceive them to see what else they may have 'missed', they may charge you again as it will need a 2nd re-test.