One last throw of the dice, try a factory reset of the hub by pressing a pin into the recess button on the rear for about 20 seconds.
If that doesn't help I think it will require further investigation by BT.
I had a web chat with BT and they are sending out a new Home Hub...as they don't know what the problem could be 🙂
I'm currently working from home, so I'm reluctant to re-set my hub at this time...in case it stops working!
Thanks for everyone's comments and recommendations. I will update the thread when the new Hub arrives.