Fuel filter, followed by a long motorway run (visit an old friend or relative) keep it steady at 70mph and resist booting it... Gently does it and let the engine and exhaust system slowly decarbonise itself with the constant heat and a steady fuel rate... As soon as you boot it, it just chucks yet more over rich fuel into the system which in turn wets the fluffy carbon deposits already in there and sticks so back to square one...
Take the manifold off and get it decarbonised properly with a blow torch.
Seafoam or similar as a worst case scenario treatment.
To prevent it from reoccurring: Use a diesel like it is supposed to be used, like on long journey's and avoid booting it everywhere when being lazy and driving to the local shop, takeaway and school run etc. This kills a JTD and lets face it.... A diesel is not designed to do this kind of work... A petrol is!
Take a look at commercial diesels and diesel taxi's that are constantly on the go doing continous mileage, Do they suffer the same issues?