Thank you so much for the replies chaps, that's a lot clearer.
So the presence of what I believe to be the oil diffuser suggests that it is a sealed system (referring to the green cylinder at the bottom of Alan's picture). And as Phil says the lack of breather on the rocker cover again points to this. However my cap IS vented, so sounds like I need to get this changed.
I looked everywhere at Beaulieu last weekend and couldn't find an air cleaner housing

Well I found one, but the pipe had been hacked off. Thinking about it maybe I should have bought it, because it had the plate etc.
So is there any such thing as a housing with the breather omitted? Or do they all have them, and it's up to me to block it off with something?
Cheers,
Matt.
'60 Morris Minor 1000 4-door saloon (convertible) with 1098 cc engine.
'91 Mazda MX-5
'99 Mercedes C250 TD