Although the Lanchester Library holds a large collection of PhD theses in print, all of which are listed in Locate, it is probably easier these days to access them online. Again, those awarded by Coventry University are listed in Locate. However, the British Library has been retrospectively digitising PhD theses for a number of years and there are now over 500,000 available via the EThOS Service.
The service is free, but you will need to register in order to use it.
If you are searching for a particular thesis, try searching for the author's last name and the name of the awarding institution, e.g. "coventry university" parkes
If you are searching for theses on a particular topic, search for that, e.g. anorexia
If you are searching for previous Clinical Psychology theses, then I suggest you try the following search strategy: