Having posted this on the forum, I now seem to have acheived a resolution, so thought I would feedback my experience.
Basically, I have connected the Swyx server into a spare switch-port on the back of the Draytek router at site. Since doing this (touch-wood), connections between the phone handsets and the Swyx server have been reliable for the last day or so. I guess that there was some interaction between the NIC on the server (HP Proliant 110 G6) and the Netgear PoE switch. I can't explain the exact reason. The Draytek router also connects into the Netgear switch, but the overall connections of the phones to the Swyx server now seems much more stable since moving the server connection from one of the Netgear switch ports into one of the Draytek switch ports.
Who knows, but I will be happy as long as it remains stable.