I'd say they have to sort it out, they took your money (which you have to pay IIRC, but do so under protest) and they have a liabilty to do the job you paid for and fix any damage.
Thing is there are a couple of acts of parliamnt in force and sometimes action under one prejudices your rights under another, like as soon as you have something fixed it's much harder to then reject it as a "lemon".
I guess as this started with a repair though it's a service that's substandard, not the goods. In whcih case I think you have to let them try again, but equally so you are entitiled to any out of pocket expenses. If they can't/won't put it right you can then go to a 3rd party for repairs and that cost would be the sum you have to recover.
But ring trading statdards, they'll tell you all the right laws to quote.