Hello StewartM, thank you for coming in on this problem.
I am very pleased to say that I have actually got over the problem and now have both my cameras upgraded to the latest firmware (v5.5.83 Build 190221) and working perfectly. I must be honest though, I really can't remember doing the last upgrade in this way. What I did to achieve my goal was, instead of going into Menu - Maintenance - Camera - Upgrade, I actually went into Menu - Camera Management - Upgrade (The upgrade button being in the middle of the screen both horizontally and vertically.
As I say, I really cannot recall doing it this way on the last upgrade but if I did, then it must be an 'age thing' as I am a grey haired old wrinkley with limited techie knowledge.
Anyway, thanks again to everyone for coming in on the matter, much appreciated.
My next goal is to try to get 'Live View' working in iVMS4200.