This problem has now been resolved and my phone divert is now working. I'd like to share here what the problem was and how it was resolved and hopefully it'll save the next person wasting hours on the phone.
I discovered that when you switch your line from analogue to digital it can be configured wrong and call barring gets activated on you line. This prevents all outgoing calls and as a divert is an outgoing call it stops call divert working. That's what had happened in my case so I rang the helpline to get them to check .. yep call barring was active ... nope they couldn't turn it off until tomorrow.
Then they rang me back five minutes later to say they had now turned it off and sure enough I can make outgoing call and my divert works again.
So, to summarise ... if your call divert isn't working then the first thing to check isn't all the techy stuff ... check that you can actually make an outgoing call from the landline you're trying to divert from. If you can't and you get a message saying " you're not allowed to call this number " then you have call barring activated and you need to contact the helpline and ask them to turn it off.
I have to say I'm surprised that I had to dig through the internet to diagnose the issue myself and then ring CS to tell them what the problem was and how to fix it but there ya go .. maybe I'm easily surprised?
Hope that helps anyone who finds this thread in the future,