Just thought I'd come back and update this story.
I rang BT with my Mum present and despite their best efforts no one was able to activate the account, and I wasn't prepared to proceed with direct debit without having some way of tracking it with BT apart from having to ring up.
So I left it until today and tried again setting up a new account as the original request had expired presumably as it didn't complete successfully.
And lo and behold, all went through fine this time and my Mum is now setup for DD paying quarterly for BT Basic landline only service according to confirmation screen (which I have printed, just in case!)
It would appear then, that the problem all along was BT's system threw a wobbly on first attempt and just needed time to "forget" the first attempt. In other words, we "turned it off and on again" which as we all know, seems to fix many computer issues.
