Hi Diggadave.
If I read your post right - you want to turn your new system into a Home Theatre PC, containing the TV Recording and the TV Playback function - together with the rest of the XBMC bits. Right?
So - there are 2 components you need to put on this machine.
1) Mythtv-backend
2) XBMC
If you don't want to do anything other than this - it would probably be a smart move to use one of the mythtv builds and then install XBMC on it.
http://www.mythbuntu.org/
Or, just go with standard ubuntu/kubuntu and install mythtv-backend onto that.
Personally, I use Kubuntu - with Mythtv-backend installed on it - and then run XBMC in standalone mode. i.e. when you're at the login prompt, you can select "XBMC" instead of KDE Plasma for your desktop. This can be an automatic login setting.
For this, just follow the standard XBMC installation instructions, or compile from this source.
If you're a newbie - I'd suggest first getting standard XBMC working (from the ppa nightlies) - connect it to your mythtv-backend using the myth:// source.
Once that is working, then get everything in place to start compiling this branch to get the enhanced mythtv connector.
I never use the Mythtv-frontend now, only XBMC - but I do still use mythweb to schedule and manage some recordings.
Hope that helps.