Yep distinctive burning plastic smell only at 1500 and above RPM. No smell inside the boat or from the engine compartment. It is not running hotter than normal, as far as I can tell - there is no alarm. It refuses to rev more than 2000 RPM but is running like always until then. The prop and bottom is definitely not fouled, it was out of the water about 2 months ago for a proper clean. This just happened now this weekend. My co-owner had a chat to a diesel mechanic today who thinks the exhaust may be blocked so will let you know what the outcome is. Any other input from you experts is valuable and hopefully reduces the bill from the mechanic ...
Check the prop before you do anything else, 2 months in a high fouling area and you might have a flowerpot or barnacle farm for a prop.