Hi fenderman, thanks for responding so quickly.
The cables runs are both exactly 25m, using pre-made Cat6 leads (which are actually wired to T568B). I have now tried two different sets of leads (different makes). I initially taped the RJ45 plug and camera socket connectors at the camera end (only intended as a temporary measure), but when the first camera went wrong almost straight away, I assumed that moisture had got in, so I chopped off the RJ45 plug, properly threaded it through the gland into the junction box, fitted the Hikvision waterproof connector and a new RJ45 plug. The camera still failed after a few hours even though the weather was dry. One camera died completely and has now been replaced under warranty and has so far worked fine outside for the past two days. The other camera is still failing outside, even though it has worked fine indoors for the past two weeks or so - connected to exactly the same PoE switch (a NETGEAR (JGS524PE-100EUS ProSAFE 24 Port Web Managed Plus Gigabit Ethernet Switch with 12 port PoE).
I admit it is a bit of a mystery, and via a process of elimination, I have concluded that it must be a faulty pair of cameras that only fail when they get cold.