Hi Steve, this works for me:
<a href="$mainimage['url']" data-uk-lightbox data-lightbox-type="image">
<img src="$thumbnail->src ">
</a>In this case I show a larger image in the lightbox itself.
Thanks Bas! changed it to:
<a href="http://dallasdesign.uk/remembermelikethis/wp-content/uploads/2016/06/miles-evans-and-chambers.jpg" data-uk-lightbox data-lightbox-type="image"><img src="http://dallasdesign.uk/remembermelikethis/wp-content/uploads/2016/06/miles-evans-and-chambers.jpg"></a>and it's now working. Cheers.