OK.... I tried this from a different angle.
I installed Squeeze (Network Install ISO) selecting only Print Server, SSH Server, Laptop and Standard System Utilities from the software selection menu.
Then I added the Squeeze repositories as documented on the Trinity WIKI.
# apt-get install desktop-base-trinity kde-trinity
Worked fine so I had Trinity 3.5.13.1 on Squeeze.
Then I changed the debian repositories in sources.list to be wheezy but left the Trinity ones at squeeze.
# apt-get update ; apt-get dist-upgrade
Did the job and I'm now running 3.5.13.1 on Wheezy.
There are some 'curiosities' (like the tdm being called kdm-trinity and the config stuff lurking in /etc/trinity/kdm where I'm sure when I tried the nightly builds it was /etc/trinity/tdm) but it is working.
Might be useful info for someone else.