Finally got round to sorting the problem -- when I found I'd 'lost' my reply button again!
Discovered it was there when I used IE, but not when I used Yahoo browser, and I tend to use Yahoo on my first visit of the day, so that I can pick up emails.
Discovered that when I was messing about some time ago trying to work out how to increase text size in Yahoo, I must have unchecked 'Show pictures' on the options menu.
Problem solved, I can now reply in both Yahoo and IE, and I'm a happy pixie again!