I finally got it working!. I'd mailed support a few times and got no help at all and the I'd already opened the firewall ports (28000 TCP/UDP, 28100 TCP/UDP, 80 TCP , 443 TCP ) and even tried it with the firewall off on both PCs and adding the PC acting as the server to the DMZ which made no difference. What did make a difference was in the settings for multiplayer adding the specific IP address of that is used to connect to my Lan. My PC runs VMware so has several IPs on different virtual interfaces.
My next issue after being able to actually join a game was immediately getting kicked out with a #10009 error. Again I figured this out by myself with a bit of guesswork and this was fixed by again going into multiplayer settings and changing the bandwidth allowed up from the default 1Mb/s up to the max of 10Mb/s for both sending and receiving. I did this on both server and client and hey presto I can finally have a local multiplayer session on my Lan. What would have been useful is having this info in an FAQ as it would have saved my hours of frustration. The PDF multiplayer guide for example is out of date as it states to fix the problem you need to download a and run a dedicated server and this is no longer the case.
At least I got there in the end