I had exactly this about a year ago and it turned out to be a corroded connection on one of the diesel injectors. The fault code pointed to a faulty injector but when they came to replace it they noticed the corroded connection, cleaned it up, including replacing some parts and the problem cleared.
I would add that my car was about 12 years old at the time and had covered around 200,000 miles. I see your car is around the same age so injector problems are a possibility/probability.
1ozzie's diagnostic is also very likely, particularly if you do lots of short trips so don't give the car chance to go through its filter-cleaning cycle. You can buy this
https://www.halfords.com/motoring/engine-oils-and-fluids/fuel-and-oil-additives/wynns-diesel-particulate-filter-cleaner-325ml-325222.html
which claims to clean the particle filter. It might be worth £10 for the gamble.