Bingo,

This is exactly what I'm trying to do, Data Protection Act and all that, sorry for the lack of clarity there. There is no requirement to un-mung the data - in fact, if it can be unmunged, its bad.

I thought this exact style of munging might be in a module already as it seems a standard thing you would want to do when dealing with sensitive test data.

I'll write a small module and post for comments.

Cheers

Kevin