Thanks for the responses guys.
I also emailed Steven Dick of Tactical Knives. He suggested 440A or 420 modified, but gave me an email address for Gerber. I'm waiting for a response from them.
If I look at all the info supplied then I guess the Mk1 is probably 440A or maybe 440C. The Guardian is brand new and was imported weeks ago, so I assume this could be ATS34?
Thanks guys, you've been most helpful. Will try to get definitive Gerber input as well.
Andre