Thanks very much for your response - you put me on the right track. I zoomed in on the logo (in a standard picture viewer) to find that there was no black line, but that when the graphic designer had converted their adobe illustrator file into a .jpg for me, the background wasn't true white.
Therefore when I made the logo much smaller and placed it on a true white background, the contrast between the two appeared as a black line on screen.
I opened the logo in 'Paint' and added a white line to the area causing the problem - it seems to have done the trick.
I tried compressing the picture which hasn't made any difference to the quality, but to be honest it looks pretty good now anyway.
Thanks ever so much for your help!!