Yes I can see how that all hangs together now thank you. In my quest for the answer to my borderline out of range idle speed adjustment screw and reading of deteriorated Breather Tower Valves, I had wondered if air from the crankcase was bleeding through the tower and running the idle speed adjustment screw out of range. In the event squeezing the pipe flat with a brake flex pipe clamp made no difference to the idle speed and sucking/blowing down the spigot demonstrated what appears to be a functioning cvs valve.
Having said that I do now have an acceptable 800 RPM idle which clearly passes my previously mentioned some might say brutal "letting the clutch up in first gear on idle and pulling away up a gentle slope" test
Got to say I do find the breather tower oil filler/oil sensor modification a temptingly elegant and aesthetically pleasing solution though.
E D I T:
So I have been using the van for local trips as a shake down exercise (Watch out for the high top in B£Q Mr Bean

) and on one such trip it started fizzling out as I drew to a halt at roundabouts etc. So getting it home by a somewhat noisy driving technique sure enough it wouldn't run at under about 1.000 RPM.
Remember my previously mentioned running out of idle screw range?
Well I wound the idle screw out about two and a half turns and all is back to normal now. Trouble is I don't really know what the original cause was/is and I don't know what fixed it

In the meanwhile I will carry on in puzzled ignorance on the basis that if the idle RPM suddenly rises I can simply bottom out the idle screw again. Some day...
Well-timed silence hath more eloquence than speech.
"A quiet shy boy who took little part in games or sport"
88 High top 2.1 WBX