Many thanks to both of you for these. The only other option seems to be to contact institutions which are known to specialise in your own field (in my case, environmental ethics) and to trawl their libraries for copies of theses produced there. Which seems rather a dated way of doing things, to be honest. I'll let you know how I get on... and I wish you well in your research!

Anyone have any idea of how to find out whether someone has done a PhD in the area one is researching - or along the specific lines one hope to follow. This seems important to me but I don't know how to find out!