Following advice received here, I think I may have shaken off the "Spitting Out Oil" Demon which had attached itself to my 350.
I started to keep the oil level at the lower end of the scale, as suggested here. I use an energy drink bottle with a nozzle to make it easier to put in just small amounts of oil, as suggested here. I removed the breather from the crankcase to the catch-can and fitted it the other way round so there is no sump in the middle run of the pipe, as suggested here.
Result - today I went to Derbyshire - a ride which involves uninterrupted runs at 50 mph or so for 35-40 minutes at a time. This should be well within the capability of the bike but I've never made this run without finding that a blocked breather-pipe has forced oil out into the right-hand toolbox, soaking the air-filter and dripping oil onto the exhaust pipe and all over the back of the bike. Today we did it!
Thanks to this Forum for such friendly good advice.
Tom