DOH!
I have found the answer already from a previous post and have solved
the problem by using an x-y chart instead of a line chart.
Sorry!

Graham wrote:
I have created a chart which compares 2 sets of data on a line graph.
I would like to add error bars to the chart, but when I do this they
overlap making the chart uninterpretable.

Ideally I'd like to move one line forwards or backwards along the x
axis a small amount bringing the error bars out of phase with each
other. I have not found a way of doing this using excel - does anyone
know how to do this or any other way around this problem?

thanks,
Graham