Problem with migrated MT 2.6 installation

I have a rather ancient 2.6 version of MT that has been 'migrated' (ie copied) to a new webserver. Clearly some bits of it are working - ie the search function actually displays correct results but on going to the MT log-in page I can an internal server error screen.

The site is at peacockshock.com

According to the hosting company tech support if they run mt.cgi at the command line they get the following errors

MT::App::CMS=HASH(0x23afd30) Use of uninitialized value $ENV{"HTTP_USER_AGENT"} in pattern match (m//) at lib/MT/App/CMS.pm line 216.
MT::App::CMS=HASH(0x23afd30) Use of uninitialized value $script in substitution (s///) at lib/MT/App.pm line 387.
MT::App::CMS=HASH(0x23afd30) Use of uninitialized value $script in split at lib/MT/App.pm line 388.
MT::App::CMS=HASH(0x23afd30) Use of uninitialized value in concatenation (.) or string at lib/MT/App.pm line 392.
MT::App::CMS=HASH(0x23afd30) Use of uninitialized value in concatenation (.) or string at lib/MT/App.pm line 392

As far as I can tell the file permissions are correct and I have looked at App.pm to see whether I can see what isn't being initialized but the numbers don't seem to match up.

My issue is that I don't want to reinstall MT and redo all the custom coding as the system uses a categories plug-in to display selected content which given that it's a 2.6 install I don't know whether I can even get hold of again.

Anyone have any ideas?

Many many thanks

Frank