Could also be as a result of crappy fuel.
I had a similar issue with my Volvo T4 and i was going through almost everything related to emissions to get it through its MOT.
In the end, it turned out to be carbon deposit build up on the seats of one of the valves.
It wasn't found until the same valve blew and i had to get it replaced.
While the head was in the cylinder head specialists, the de-carbed the head and lapped the remaining valves for me too.
Afterwards, when the valve was replaced and the engine put back together, the emissions were as clean as new car with an engine of similar size.
I have a similar issue with my Range and now that one, or both, of the head gaskets have gone, i will ask them to do the same on them too.
The emissions test for the MOT barely scraped through but seeing the carbon build up on the valves, i can understand why.
Once i get the other head off, it'll be interesting to see the build up on that.
I expect it to be pretty uniform.
I'll get the emissions re-tested once the engines back together and i'll let you know the result.