Okay so I've had another look at things.

I still cannot change the speed on either the virtual panel or the MCP.
I've tried both connected and wireless mice (free range) but still no joy. I can change the altitude and heading but either left or right clicks but not at all my scrolling.

Just to make completely sure I've not got the mouse in any form of zoom mode I've cleared all the menus for views etc so the mouse doesn't appear anywhere.

I'm kind of running out of ideas - what are your thoughts?

I really appreciate your help!
Adrian