@DevdattaTengshe and Brad and PolyGeo thank you for your comments. As strange as it might seem, I'm interested in the general process of using non-ESRI data formats, rather than a specific format. The answer from Nir in the comments would be fine from my point of view if it could be fleshed out a bit as an answer. I take your points about the question being too broad with regards for the terms-of-reference of this site - but on the other hand it was the question I wanted to ask!

I have clarified the question (hopefully).

Although this works, I have supplied another answer, which I prefer, and accepted that one.

For me, the line of code would read: mapControl.ActiveView.ScreenDisplay.ScaleContents = True.

Sounds good - do you think you could provide a code example in your answer to make it really clear?

I'd +100 this if I could - very helpful intervention, thank you. Works very nicely for me.