Thanks everyone for your thoughts!
I did some investigations yesterday and it appears to be a problem that I am 'setting the mark' by clicking on a spot on the screen and moving the cursor a little.
In more detail - it seems that this happens more readily when the mouse is beyond the last character on a line. If I have the mouse over characters but away from the cursor, it seems that the drag event is less prone to triggering. I don't quite understand this because I don't think the drag event is related to the state of the buffer directly under where the drag occurs. I suspect this has become more noticeable to me recently as I've now started actively managing my init file and cleaning up what happens when a buffer takes on a mode. In particular, by cleaning up my prog-mode settings, I think I now have some whitespace cleaning going on that wasn't previously. I suspect the fact that I have less space characters at the end of my lines is what has brought the issue to prominence.
I think the solution is for me to disable the drag to mark feature unless it is done with a 'hot key' depressed. Interesting take away for me - the issue was caused by the fact that my settings are getting more organized so things are working rather than because something had stopped working. Once I have set that up, I'll update this post for future generations!