Thanks for the replies Gentlemen!

Duke Johns point about the valid animation for the model might well be it. My undead warrior was based on a peasant model, and the bone naming and detail layout may well differ. I will re-run my steps with that in mind!

Mr Egg ... Short answer is yes....with a bit of No ;)

I extracted all the animations from the DAT file and ran the game to check everything still worked.
Next I used the animeditor tool to create a new animation which was a COPY of an original one. It was basically just a re-name. I amended the text file to look for it, and that ran OK. This confirms to me, anyway, that the basic tool is not at fault....but rather my application of it!
The last step was to change one of the animation CAS files and update the DAT file using the batch file that the tool creates for this purpose. This was when I got the hard-lockup.

What I SHOULD have done here was to create my copy animation and run the batch updater WITHOUT MAKING A CHANGE to confirm that this wasn't breaking the DAT file. Will do that tonight :D

I will let you guys know how I get on!