Thanks, your comment about multicast put me on the right track.
We run pfsense on KVM and used the passthrough NIC setting on the guest rather than just using a bridged set up. For reasons beyond me, this does not allow multicast traffic to pass to the network. Changing to a bridged network config for the pfsense VMs solved the issue. omping for the win.
My other issues are best placed in another topic, so marking this solved. Thanks!