Have you failed on emissions?
Or is the MOT tester looking at the cat and just saying "that's wrong"?
As mentioned above, if it's a racing cat then it should pass as it should be working as a catalytic convertor, therefore reducing emissions.
If it's just a straight through piece of pipe then it may well mean emissions are above the allowed levels.
But if the MOT tester's just saying "it's too loud" he's just being an arse - take it elsewhere to get done.
Don't buy new from renault, at £1100 that would be like getting dry bummed. Source a secondhand one from a scrappy or check the for sale section in here. Or it could be worth a call to MATT BLACK or Yozza to see if they've got one lying around they can post.
And if you have got a racing cat, don't just give it to whoever swaps the new one on, keep it!
As a tip for passing emissions, I hear that putting a bottle of injector cleaner in beforehand (the one you put into the petrol, like Redex) will help things, as will asking them to do the emissions test at the start of the MOT when the engine is still at its hottest.