Thanks to all who replied.
This tale has a tragic end. well it would be the end.. if I wasn't still cleaning up. But as a warning (or amusement) to others who may follow in my footsteps... I shall share my woeful tale..
We decided that an in place upgrade was the simplest and most straight forward approach. (in part due to responses provided here.)
For a variety of reasons ( I wasn't present) , the in place upgrade was aborted, and they (who shall remain nameless) performed an un-install of 2000 and install of 2005.
They reattached the databases and then bodged their way through the app upgrade, and got the app working sort-of...
The following predicted things happened:
1. Server logins were lost, passwords reset/changed
2. Linked servers were lost.
3. Automated database tasks & SQL agent jobs were lost.
4. Some stored procs were lost or broken.
I so I have spent all day playing SQL-wacka-mole.
What I wasn't expecting was that the new server was installed with a different collation, from other SQL 2000 databases, which meant that once the logins and linked servers had been repaired, SP's containing queries which used remote tables in joins failed.. took me a while to fix those...
Last but not least, as we had to reset a few passwords to get things working (we didn't know what the old one was) a few over looked database connections and applets have now stopped working as they contain hard-coded passwords
So it's 9:00pm... and i'm currently on the phone working with a developer who left here more than a year ago, trying to unpick their work and revive their app...