it seems the youview box may have lost its IP address when infinity was switched on (several weeks after the homehub5 was initially delivered and set up).
Although, the youview box was connected using an Ethernet cable directly into the HH5 - I also had an additional Netgear router in another part of the house which was connected to it via Flex extender plugs. The Netgear router appears to have 'hijacked' or confused the IP addresses when the broadband signal dramatically improved. (from 3.5 to over 60 Mbps download speed - wow)
An older ipad2 happily stayed connected throughout to the HH5 when nothing else would. So, I have also changed the HH5 wireless SSID names to separate 2.4GHz and 5GHz, and now an iPad pro and iPad air are holding their connections solidly when connecting only to 2.4GHz.
Thanks to the forum and Robbie for helping me thru this.