Hi,
I managed to resolve the issue for our case in the end.
The two servers we're using as our pfSense boxes are Dell PowerEdge R210II servers, each came loaded with 2 on board Gigabit Ethernet ports (one being used as the WAN interface and the other for the LAN interface).
In the first instance I had setup the pfSync to use the LAN interface, which I'm led to believe is a big no no, so I then set up a separate VLAN for the pfSync to use, but as this was still using the physical adaptor shared by the LAN interface, it made no difference.
In the end I bought and fitted an additional PCIe Gigabit Ethernet card in each of the servers, set up a VLAN to use the new physical adaptor (not being used by anything else) and set the pfSync to use the new VLAN and since then I have seen no issues with the sync slowing down or the Backup box becoming unresponsive whilst adding users.
I have now put the new pair into production and we've seen no problems.
Thanks everyone for their help and suggestions.
Hopefully this will help somebody else encountering similar issues.
Cheers,
Jan