I pick up 6 packs of 25g or 33g single bags in Home Bargains when I am in the north - Surrey/Sussex is too posh for Home Bargains sadly. However, I haven't seen 50g bags in shops, only on trains!
Perhaps they are reserved for us Welsh. We are renowned for liking salt, and I find Godlen Wonder the saltiest crisps by far, at least in taste.
Anyway, back to to the trains, do any TOC's offer Yorkshire Crisps? Their website says that GC will offer them on their Bradford route - including Henderson's Yorkshire Sauce flavour!