Hi everyone & thank you for all your replies and advice!
I am not very pc savvy and it is with some embarrassment that I admit I finally worked out why I could not use the Contact Seller function to send messages.
When presented with the small :Contact Seller" box on the right side of the window, there is an upper "Send Message" in small writing and below it another "Send Message" with larger writing. Initially, I placed my cursor over the upper "Send Message" but there was no indication it was a clickable area/link so I moved my cursor to the lower, greyed out "Send Message" that was clearly a clickable area but it was not responsive to my click.
The first responder to my question actually gave me the answer only I did not realise at the time. It wasn't till contessalmirena 's reply "You should be able to type into that field, even though it looks counter-intuitive" that made me take another look and this time I clicked on the upper "Send Message" (which does NOT look clickable) that revealed a typing box.
Perhaps the upper "Send Message" writing should be replaced with "Click here to type message" for people like me. : (
