Well I got a call from Geoff on Saturday and it looks like the diagnosis is over and the re-build can begin.
Having lifted out the engine, and having found the oil pump to be okay, it was a case of turning the engine @rse side up and getting into the nether regions.
The photo's below show clearly that the crank shells are worn, and at the flywheel end are worn through to the copper, which explains why there was metal 'dust' trapped behind the filters.
The photo above also shows that one shell has been scored, but thankfully not at the expense of damaging the crank as shown below.
So all the crank and big end shells are to be replaced and the crank itself cleaned, and given that they're in there anyway, all timing chains replaced.
All in all a painful experience (I haven't driven my Maserati in nearly two months and I'm getting withdrawal symptoms), but god forbid, if I hadn't caught it in time I could have blown the engine, and once she is returned to fine fettle, all the main wear points will be replaced and she'll be ready for another 70K miles
Not looking forward to the bill though.