I almost can't believe it as I just don't understand how it could have been causing the issue, swapping the router has resolved the problem 🥳
Thanks @licquorice @Keith_Beddoe @rbz5416 for you advice and support. 👍
I am pleased you have it sorted, you are not the first to have problem with VPNs on the home hub.
That router also has QOS controls, should you need to allocate a higher priority to some devices.
Question now is whether the Smart Hub is faulty or has a generic issue. It might be worth trying to get your hands on a SH2 to test, bearing in mind that if you're moved to Digital Voice you will be forced to use one. Would give you time to plan & test options before potentially having to make a choice between a working VPN & a working landline!
As it happens, following my very frustrating call with BT 'support', I am expecting a new Hub6, so I'll be able to test and see if it was an issue specific to my Hub6.
I will let you know when that arrives and I've tested it.
Bear in mind it will likely arrive with old firmware that will automatically update at some point. So if it works out of the box, don't be in too much of a hurry to return the the TP-Link until you've seen the firmware updated in the log.