Well, I’ve just figured it out and it’s the exact same problem as I’ve had before (took a while to remember it but it’s just come to me):
When replacing a working firewall on the same network (which is the same thing I did previously) with both firewalls pointing at the same NIC on the same server, only one will work - that’s the one that the default gateway is pointing to on the server NIC. I don’t know why this is even a thing, but I’ve changed the default gateway to the IP address of the IPFire box and it’s all working as expected.
Thanks for everyone’s help.
Dave