Thanks for your reply Bob.
Yes, we did have the injectors serviced and tested at 1140 hours, as per the Yanmar operation manual (at a large boatyard in the Bahamas, which is another story). Interestingly, it was later that season when light smoke first appeared.
It certainly looks like your injectors are performing better than mine. I believe DEE-ZOL is quite similar in its performance claims to the Stanadyne product that I'm testing. Have you seen other benefits from this product, e.g. better mileage etc.?