Many thanks for your reply.
I'm aware that calll divert and voicemail are incompatible. I therefore wish to cancel free voicemail on my landline and activate call divert. Voicemail is definitely active on the landline but attempting to disable it in order to activate call divert is providing to be difficult as the option to do so via 1571 is not available.
I have been given to understand that a settings option should be available via that route, but no success there.
Attempting to implement call divert produces a recorded message that call divert is not possible on this number, and Customer Services afirm that voicemail is extant.