OK. Thanks everyone. I've seen the SSL site, which indeed looks as if it has the right bit(s) of kit. I had not thought that bleeding might become quicker, too: that has also been quite difficult on occasion, so I can hope for a bonus...
On our engine we have a CAV type filter with aluminium bowl and off-centre drain plug. The bowl is big and more or less rests on the gear shift cable which passes beneath it. This makes getting it off to change the filter a real bore. Could I just swap it for a shallower bowl? Or is there...
Well, I've cleaned the carb as best I can. (When I first said it was clean, I was referring only to the bowl.) Found a tiny bit of Verdigris (?) on the needle, but otherwise all was as bright as bright. And lo and behold: starting now seems to be reliable and predictable. Bingo!
There is...
Having been very well-behaved for years, our little outboard has developed a grumpiness. Advice would be welcome.
Full throttle and choke may produce a start, but re-starting is very difficult. Plug is new, fuel reaches carb easily, carb is clean, fuel is not very old - but the symptoms...
Just a word of thanks again from the originator of this thread. Solenoid was my problem: quite rusty. Revitalised by a good chap in Botley. Motor itself reported to be in surprisingly good nick for its age. Removal and re-installation needed two confident people, so I did not (much) mind paying.
And just to complicate the issue, I don't think I have enough hands to make these connections AND turn the starter-key.
So far, I have had no opportunity to act on any of this very helpful advice - but I am grateful all the same.
Ok: that's easy: thanks, Piers. The only trouble is that I won't necessarily know whether the clean-up has done the trick - unless I wait for a non-starting incident and then do the clean-up and then discover that all is well. And even that would not be conclusive, I suppose.