Thanks for all the [prompt] responses
New tool on me - Info > Audit Trail clearly shows boths students IDs on one file and only one on the other - bang to rights.
Also, opening the files in notepad and looking at the Header info.
For info, this is part of a reasonable size submission from 2nd years, the zero mark I will give will mean they fail the module and they will have to face disciplinary action - so I needed to take it seriously.
Alright!! Plagiarism not plagarism - after 8 years teaching this lot CADCAM this is only the 2nd time I've had to use the word!
Cheers, Sean, D&T, Loughborough Uni