I immediately contacted Royal Mail via their online system and noted the problem and my concern that this could mean the stamps had been replaced with fakes before reaching me. I asked if there was any way I could verify that they were legitimate before I used them.
A few days later, I received a bizarre email back from someone at Royal Mail, saying they were sorry to hear that I wasn’t happy with their response (what response?!) and can certainly understand my frustration but we are not able to progress any further (than what?!), along with various other words that seemed designed to try to appease me while not actually saying anything of substance, and a recommendation to contact some other part of Royal Mail, which I may yet do.
It was a surreal experience, but having been prompted to do it by seeing several reports of people who’d sent their stamps in and either never received the replacements or had problems with them, and having received such an obviously tampered delivery, with the entire process from collecting the stamps from my local post box to delivering the replacements to my home being under the control of Royal Mail, I can’t say I was particularly reassured by the response so far.
I haven’t yet tried to send anything using the new stamps…