Oh my, My petrol tank was also a fine Maraca.
Not only that, but the filter in the tank was also burnt and melted!
on top of the tank was a slight welding repair. DIY, but actually does the job fine.
I'm thinking the welding or preheating of the tank to burn off fuel melted the filter and no one noticed or bothered. This along with bits of the weld, rust, shingle, etc.. seem to have been the culprit(s).. unfortunately my pack of gaskets for all eventualities- didn't include the carb.
so, .. before discovering if the wee van will drive, I have to try and cut out a replacement from a sheet of gasket paper - which I shall attempt today.
the tank and Zenith 281F carb are sparkling clean now, and I have decided to add another inline filter between tank and pump.. pic and updates to follow