Thanks you to all those who posted answers to my original question a few days ago. Sadly none of those suggestions worked. In desperation and because of another issue I re-started my laptop today and hey presto......I have arrow key scrolling back! Why oh why didn't I try that earlier?!
This might sound a silly question but did you remember to click in the window before trying to use your arrow keys for scrolling? They won't work if, say, you've clicked on the taskbar (or elsewhere outside the relevant window) immediately prior to trying them; the window needs to be 'active'.