This car does not have a DPF, so that is not a possible problem. More likely to be the EGR valve sticking, or one of the many sensors (mass air flow sensor comes to mind as a reasonable suspect)

However, you could just go and replace those parts, and find it's something else entirely. Far better to get a diagnostic test carried out if possible, rather than random parts replacement.

Regarding diagnostics, look for a recommended independent garage if possible. HJ carries a list of customer-rated garages, and there are a couple of other systems too (goodgaragescheme.com is one, sure others out there). Look up, find one local, read reviews, and pick one.