@Solla-topee 

Again, thank you for the reply. Curiously, after a day of behaviour as I described, it appears to be working correctly now.

Indeed, the whole system, both in terms of response and signal strength, appear to be much more stable than when first installed. 

Whether such 'settling down' always takes a little time, or there are other factors, will likely be impossible to determine.

The only change made in the period in question, which might be a factor, is that I was replacing a Yeskamo system and that the NVR base, together with two cameras, was still in operation until the final 'handover'. I can't see how the wifi/signals of that system would interfere, but it does seem more than coincidental.

Even if you can't know whether such interference is possible, it might be worth bearing in mind if other users are experiencing odd behaviour of their cameras.