Hello, Richard.
How's the flow of oil to the rockers?. I know that the B31/33 doesn't have a feed directly into the valve guides, at least my B31 doesnt, but relies on what splashes around after its been through the rockers. Try slackening the feed pipe banjo bolts when the engine is ticking over to see if some seeps out. Putting your finger over the oil return in the tank will force more oil up to the rocker feed so you can check it isn't blocked.
You could also check that the banjo bolts have been put in the right order as they do have different sized holes. Smallest for the inlet, next size for the exhaust and the biggest for the drain pipe at the back.
My Matchless single inlet valve has a separate feed that can be metered by a pilot type screw on the head. Helpfully, the book says if you hear the valve squeaking then increase the flow of oil
Did you hear any sort of chirruping noise before the valve stuck?
As Julian says, it could just be a build up of carbon, so a head off job might be the only sure way to find out. It's not a difficult job and can be done in an afternoon.
Steve