In case it helps anyone else, I eventually found the problem.
Under normal conditions, when Rexygen studio is started you get a dialog box asking whether you want to create a new file or open an existing one: In my case that dialog box was not displayed and the main window was frozen.
While trying different things, instead of opening the program directly, I tried double-clicking on an existing MDL file, the program opened and functioned normally.
I then closed the program and reopened it and this time I got the dialog box and now it works perfectly.
I can only assume that for some reason Windows was placing the dialog box behind the main window, as the main window is blocked until the dialog box is closed you cannot move or minimise it to expose the dialog box, somehow opening an existing MDL file directly unblocked this situation.
