Postscript which might be useful to others...
After replacing all engine throttle/gear control mechanism and cables
, contemplating the possibilities of gearbox
and propellor causes, I had the injection pump removed and sent to Bosch in Lisbon for stripping, cleaning
They found the pump was clean, but that various parts
within the pump were worn beyond tolerance. THey replaced these and rebuilt the pump. It was their view that these worn parts
could have caused the problem that I was experiencing.
I have run some considerable distances under engine since then and no further problems.
Bosch expressed some surprise that the pump had been 'apparently' sent for rebuild
in 2007 (when the whole engine was rebuilt by a reputable UK Volvo Penta
Anyway, credit to Wavetech in Cascais, Portugal
for their diagnosis and speedy turnaround. And thanks to the forum, of course.