Well good news and well, not sure now.
Carb o-ring in place so that's not the issue. Took the back plate off and hmm the gasket is not ripped BUT is every so slightly deformed (ie bent over on one side by a mm or so. MIght be enough to let air in at that point.
In any case, I've flattened it back as flat as I can get it. Both mating surfaces and gasket have been cleaned and just awaiting drying now (probably is now as brake cleaner evaps really quickly).
Will try once again (without RTV for 1st attempt) to start. I'll be back.
In a related question: anyone get glow plugs that are dead right out of the blister pack (or really soon thereafter)? My original won't light up so that's not a biggie but when I try to light up the new one (just in the igniter, it won't). I did get the engine to fire up when it was in place but engine died when igniter removed. Hence my question.