That's annoying!
It's got to be a blocked filter, breather or the pump as you say.
My first thought was a dirty injector(s). That could manifest after a storage period if some gum or muck washed through with the new petrol?
The "tea bag" filter on the pump pick up takes a lot of blocking, so I wouldn't suspect that. Have you tried running it with the filler cap just cracked open?
It could well clear itself after a good run if you could keep it running long enough to do so.
I can only suggest reading Capo's explanation of the fuel system, which is a fine tome of info.
The section about the pump voltage and how load effects it might be of particular interest?
I'd be tempted to put a meter on it with the engine running if that's possible. To see if it does run when the rail needs pressure.
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