Possibly, I had some kind of similar problem with my TFN here in Australia and eventually the ATO gave me a new TFN. And just recently, the ATO blocked my access to My Gov for some reason, possibly linked to the earlier problem. Perhaps the ATO and HMRC "talk" to each other?
"We've got a suss one here - lives in one country, income from both, already had one problem with possible fraud."
I tried to clear my cookies on my phone and it seemed to work but I'm still unable to log on to HMRC. I have always used a tax agent for my Aussie returns but done my British ones myself.
@AlmatThank you! I will PM you my email address. Really appreciate it. For clarity, there’s a section in the application where you provide the names of ‘Supporting Witnesses’. Should these be the same folks as those that fill out the 888 forms? It seems to ask for details of 2 supporting witnesses, but I see that you can add more.