Hi pianokeyjoe and giano. Many thanks for your replies and advice.I am very nervous about taking the board apart (I did open it up when I bought it because it needed a good clean ) There was dirt everywhere inside and I am hoping that its just some dirt lodged under the keys. The keys involved are fairly high up on the board (5th octave C and Cblack key) but just knowing that they aren't functioning correctly just niggles away at me. I am busy just now...but maybe in a week or two I'll open it up again. Thank you both again for your help.
Ernie.