I just finally worked this out and thought others may find it useful someday as Google failed me....
I couldn't get lightbox working due to silverstripe's customised prototype.js (the conclusion I came to anyway) so I used thickbox
PHP Code required:
Prior to this I was running into errors such as $(domChunk) is null which in IE is Error: 'null' is null or not an object on Line: 22 or "$ is not a function"
It is all about $ used by both prototype and jquery. Unfortunately jQuery.noConflict() doesn't seem to work with ThickBox.
It should be possible to use the built in greybox (or whatever it is). Feel free to share your experiences with modal windows.