I don't think you are going to find out without having the codes read. I would suggest a diesel specialist rather than a garage, they usually know what they are doing.

Make sure your air filter's clean.

If the engine management light ever comes on and then goes out again a few days later, then read the code and it's likely to be 'EGR performance'. The intermittent smoking problem can possibly be an intermittently sticking EGR valve - without giving a fault code.

Run it with the often-recommended 200:1 shot of two stroke oil at the next fill-up and try a shot of Millers Ecomax as well. It keeps the smoke down on mine and that's done 188K miles.

Pottering around in it for a few days will cause a build-up of soot in the exhaust, so that if you have to gun it uphill -- motorway entry slip for instance to get up to speed -- that's when the build-up will be blown out and leave a cloud behind.

Only other possibility on these, because they are quite simple compared to today's model, is the state of the injectors. If they're noisy, they will need new seats and seals. Leaking injectors could cause dribbles of diesel into the cylinders with smoke as the result. This engine is known for leaking injectors. Again, the Millers will give 'em a reaming-out if it's a mucky nozzle problem.