there is a main bolt at the top of the cluster but when unscrewed the cluster does not slide out, l assume there is another fixing somewhere underneath

Yes u r right. Once l wanted to put a replacement bulb in mine but was finding it difficult with the lack of space. l thought I'd take the cluster out, so unbolted the bolts that r visible underneath the bonnet. l then pulled the housing, but it seemed to be stuck into the bumper so l gave up & after trying to exchange the bulb with cluster in place found it was not so difficult after all. Suggest if u r replacing a bulb to give it another try with cluster in position, once u find the clips even with lack of space they do come out easily.

If u must exchange the cluster because it is damaged or something then u will have to take the bumper off. There is some information on that in this link.

http://cliosport.net/forum/showthread.php?t=181155&highlight=headlight

Good luck!