It seems that despite my previous experiences with TMG 2010, I still stumble when creating a TMG array. Here are some “notes to self”, which will hopefully stop me making the same mistakes next time
Get the NICs right first In this case I came to a project after the initial installation of the array and there was no dedicated intra-array network installed. I added a new NIC to each VM and configured the IP addressing, VLANs and routing, but could not get the intra-array network to ping, let alone talk to each other.