Many thanks, gentlemen, for your kind replies. However, I'm afraid I couldn't understand how to get either method to work. Sorry I'm so thick!
Anyhow, I've now managed to work out my own way of achieving the desired result. Firstly, I deleted one of the 2 "Horn in F" staves and edited the remaining one under House Style to read "Horns 1 & 2 in F" I then went to Create,instrument and from "all instruments" selected "Horn in F(bass clef)" On adding this to the score I just had to edit its name to "Horns 3 & 4 in F", using the same procedure as before. Finally,I went to Create, Clef to substitute a treble clef and, BINGO it worked! :)
Being new to Sibelius after so many years of easy-to-use Noteworthy,I feel rather like a novice chess player attempting to pit his skills against a grand master!
Cheers,
John.