Mechanic trouble - advice needed - England

Hi All

So i recently took my old car to a car garage for its MOT. It also turned out it had a blocked dpf. Mot bill was just under £1300, with £280 of that on the dpf. They called me, I paid over the phone. They close earlier than I finish work so I asked them to leave the keys on one of the wheels. Upon turning on the car in the garage car park, it instantly flashed up saying that the dpf was blocked. As they'd all gone home, I rang them the next day to tell them. They told me to bring it back in a weeks time and they'd have a look.

After dropping it to them for the second time, they rang me saying it needed 2 new parts for the dpf. A vapouriser and an inline fuel pump. The guy on the phone suggested that to keep costs down, I buy parts from a breakers yard on eBay. Sure okay, he even looked for me and emailed me links to the right parts. Nice guy. I ordered the parts and once they got delivered a few days later dropped them around to the garage. The next day they rang to say it was all done. Another £285 quid to put the parts on.

I picked the car up and that was that. Didn't really drive the car for a few days after as I own the motorbike and the weather was too nice to not be riding. Roll on the weekend, I had to take my dad to the next county over to drop his truck off at a specialist company to have the underbody sprayed and protected. Its about a 3 hour round trip, most of it on dual carriage way or motorway. I drove with the car in slightly higher revs to make it do a dpf regen. 25 mins in it flashes up that the dpf is blocked. Great.

Monday morning comes and I ring the garage to say that its STILL blocked. 3 days of hearing nothing I finally rang them to find out whats going on. Im informed that unfortunately its either going to need a whole new dpf, or pulling apart and cleaning. Something they cant do but know someone who does locally. The guy on the phone said he reckons that would cost £250-£400.

My question is, should I really be paying again? They've had it 3 times and ive paid over £550 to them to fix it. Not to mention another £150 in parts I bought off Ebay. Are they now not obligated to put this right without me paying for it?

Sorry for the long text wall, and thanks in advance for any advice.