I have been out for most of today and I took the opportunity to try to connect to my system via my 4G phone.
I'm pleased to say that everything seems to be working fine. I didn't even have to 'switch' connections, it just logged on to my NVR/Cameras automatically.
I have also got iVMS4200 working perfectly on my pc and Hikvision is spot on too using Internet Explorer.
A million thanks to you JB1970 for your patience in sticking with this 80 year old 'not-too-techie' wrinkley. I really do appreciate your efforts on my behalf.