I want a lightweight 'head band' type microphone for input into my PSR 8000 - can anyone suggest a make / model that would give good input for use on the vocal harmony function of this superb keyboard . . . ?

Hi,
Very few head-set mics have much in the way of output quality. I would say try to keep clear of the cheap ones as they are OK for backing vocals but just don't have any power or depth to them. Audio Technica do one for around £199 that performs well. Other wise, you will have to go for AKG. (I'm not a big fan of Shure mics)