Usually avoid Plumb Center like the plague, but needed a plumbing part & was only around the corner...
During the eternity I waited to be served I noticed a large poster with the headline "Show us the card or we'll show you the door".
Poster was clearly a joint effort between Gas Safe & Plumb Center, the strongly worded message was that they wouldn't sell 'safety related' gas parts to anyone not carrying a Gas Safe card. No details of what 'safety related' gas parts where.
Looks like jobs for the boys to me.
... and they didn't have the bit I wanted - a bottom entry WC float valve.