520d 2009 dpf filter replacement help

[jamphi24]

Hi,
I have a BMW 520D 2009 estate. This week it suffered a loss of acceleration power. I spoke to my aftermarket warranty company who told me to take it to Halfords autocentre.
After investigation they have said basically the DPF filter and breather valve need replacing (they are not working which is stopping the turbo in turn from working). My warranty company is saying this is not included (does this sound normal?) and Halfords want to charge £750 to replace with an OEM one.
I have seen that eurocarparts sell an aftermarket one for about £250 which is obviously a big difference! I asked Halfords and they say that the aftermarket ones generally don’t work. I can’t seem to find much in google about them.

Car has done 130k miles and not sure if has been replaced previously therefore thinking a replacement is the only viable option'.

What are peoples thoughts?

Thanks
James