Well just read the comment about XP only being on 2228, yet after the reboot for that update the avast UI > Update showed 2229 was available.
It installed and requested a reboot, so I'm just about to reboot, fingers crossed.
EDIT: well the update appears to have completed successfully and has been rebooted. This appears to have stuck so the XP install test for this beta build appears to have failed.
Image 2 and image 3.
EDIT: 2 now I'm getting the Your Avast antivirus is outdated popup. I guess it is now going to correct its error in installing 2229. Just about to find out, but if this is the way the check for XP is meant to work that is ridiculous (but I suspect not).
EDIT: 3 Well that is what it is doing downgrading to 2228 (image 4), I hope this isn't the way it was meant to work as this is pretty damn poor, install first reboot and have to downgrade and boot again.