to think the cause of all the illness at the Wahaca restaurants in London must be food poisoning, not norovirus?

[bibbitybobbityyhat]

Why would several branches be affected if not?

And surely it means there's a problem with Wahaca's suppliers! Have they been shut down too?

Public health England say Norovirus.
It would be easily spread from one restaurant to another of staff were working at different restaurants e,g to cover sickness of those off with norovirus!

Norovirus can live for a long time on food. But you're right - it could have been contaminated high in the supply chain then distributed to the various restaurants. Contamination often comes from a food handler who's been ill. But if PHE say it's noro it probably is - this is their specialist area after all. They may have had detected it in a sample from one of the sick people.