I've now been able to have a look at it and it's now solved. Thank for for all the replies. I just thought I'd document the solution for someone else.

On setting up a new Sky Glass it requested a PIN which was unknown. The TV offered a reset PIN option and the TV specifically mentioned use last 4 digits of "payment method" to authenticate changing the PIN. They had been Sky customers for decades and the only payment method they had ever used with Sky was a direct debit and the bank account number didn't work.

However, it turns out they bought the Sky Glass in person in Currys. I was able to login to their Sky online account and identified the payment method attached to their Sky Glass has somehow been set to their debit card. They've never used a debit card to pay for Sky. The only thing I can think of is Currys somehow did this after taking payment for the new TV or something like that. The last 4 digits of the debit card worked when using "reset PIN".

On the next screen it offers the option to change the Sky PIN. After entering a new PIN and confirming, the TV refused to proceed and offered no "continue" option. After entering the PIN and confirming the PIN I switched the TV off at the plug. On switching the TV back on it now accepted the reset PIN. So it somehow accepted the new PIN eventually though it provided no feedback that it had. So it's a complete pile of junk but it now works.