All,

The problem turned out to be the transmission oil cooler which was 3/4 blocked.  The gurgling has stopped - I'm pretty sure this was the exhaust anti-syphon pipe sucking in air because of the upstream blockage.

Thanks for all the advice, really grateful.

Interestingly, now sea water can flow at full rate, when I rev the engine to over 2,000 to engage the alternator the sea chest alarm sounds, so at least I know that works!  I'm hauling out next week and will give the sea chest inlet a thorough check and clean out.
--
David Crisp
SV Wilna Grace
Amel 54 #58