A long shot - an elderly relative has this model from 2007 which works perfectly except that it goes into standby after 30 mins and then turns itself back on after about a minute.
I suspect it is some kind of lock but can't find a user manual anywhere to confirm the keys on the handset. Any thoughts, please?

Sounds more like a failing power supply. A capacitor failing would give this kind of problem. Given its age its not worth getting it checked out as a replacement will only be about £120.