DPF is a static filter, which is regenerated by various processes.. if the regeneration is not working properly the filter will end up more blocked than it should be. This would result in loss of mpg but wouldn't result in more smoke, so the MOT would not pick it up.
The car in question is a 2006 VW Passat. And I suspect that fault codes don't *always* appear on the dash, I suspect they just get logged for the garage to read about at the service.
Some of my friends go along that route, often ends up with something failing big time that they have missed.
Mmm.. yeah.. this is what I am thinking.
Garage it is then I suppose.