Renault Van MOT and cambelt

I booked my Van in for an MOT, (renault kangoo 1500 DCi diesel) and a new cambelt. Originally they refused to do my cambelt but when I rang up they said they could do it. It turned out I needed a few repairs e.G. Front brake pads etc bringing the price to around 352 pounds.

I got a text that night saying my van was ready to be collected (and working). I went to pick it up the following morning and it was still on the ramp, and they said they couldn't get it started. It turns out they fitted the cambelt and the top pulley broke because they didn't quote me for or buy a new one this has also bent some valves (apparently, this is what the mechanic told me on the phone). So now they are asking for another 250 pounds on top of the original 352 (which I couldn't afford either) something was also mentioned about a woodruff key but I didn't understand half of it.

In my opinion this is a breakage, that is due to the improper installation of my new cambelt. I didn't have any problems with the old one, I just thought it was due a new cambelt as it was currently over 80,000 miles.

The mechanic has offered a "compromise" which involves the labour for free if I pay for the parts. But I beleive there would be no need for parts if they had quoted me for the correct parts initially and installed them properly.

should I pay for this extra work? I feel like I have to pay for the mechanics mistake and I cant afford to pay this!