Thanks Rog, very helpful. Your pics & explanation match what I can see. As mentioned, the return spring is so weak it doesn't do it's job, but when I removed the rocker cover I could see that the lifter wasn't holding the valve open, so shouldn't have done any damage. I poured a bit of petrol into the gap above the valve, and it did disappear so there is a leak, but after I'd taken off both rocker covers, and top and bottom inspection plates to establish that both valves and tappets were moving freely, I bolted all back up again, gave it a hail mary last kick before giving up for the day and bless me it started first time. Maybe the exhaust valve was a bit sticky and the petrol cleaned it up a bit, who knows. Anyway, it's LOUD, way louder than my B44, but ran sweetly.
Needless to say, it wouldn't be a BSA if the ending was perfect, because there's now a large pool of oil on the drive. It's dripping from the left hand side behind the primary drive cover. Funny it's only started since I started the engine, which makes me think it's engine oil under pressure, but what would that be doing in the primary drive?
Photos to follow when my email decides to behave, the wonders of a 2 Mb broadband connection