Aftermarket DPF?

Bought a second hand diesel car from a dealership back in June. DPF clogged three times in six months despite long and fast drives weekly. Took it back to the dealership once for the smell of fumes in the car and was told there was a hole in the exhaust which they patched up, then again for the DPF which they cleaned out, both under warranty (which has now ran out). Not even a month after the second time the DPF is clogged again. Took it to a local garage and they said the DPF is an aftermarket part of poor quality so will repeatedly get clogged unless replaced, which looks to be about £600-1500. Is there anything i can do in terms of getting a partial refund from the dealership? I’m not sure if they had to disclose the aftermarket part in the first place, but the fact i’ve taken it back twice and both times they haven’t told me about the part is making me wonder. I’ve also been getting notifications to change the oil even though it was changed along with the filter when i bought it, not sure if this is related? I don’t know much about cars at at all so any advice would be much appreciated 🙏🏼 Thanks!