As a singer Les, you will have already developed all the skills for coping with nerves - visulizing the audience without clothes on, focusing on a good listener, deep, slow breathing beforehand whatever, but playing an instrument is a whole different matter isn't it? A friend of mine had so much trouble, as a beginner, in remembering tunes that he became an excellent improvisor. He knew his instruments so well that he could do amazing things with the tune until he found his place again. Nobody ever knew how many times he'd played a B part and he always forgot himself - he just found his way to the end somehow and wonderful things happened in the meantime. He could even do it within a group. You still have to cope with the nerves but if you are confident about being able to find your way out of the woods, there's one less thing to worry about.