I have supervised three PhD and three MSc students who all successfully completed their degrees. The last of the former has now successfully defended his thesis.
I always welcome enquiries by potential PhD students. However, since my research is mathematical in nature I am looking for students who have a background in mathematics rather than computer science. While modelling programs and proofs may sound like applied mathematics (and it is) the methods used are typically more familiar from pure mathematics. I am not too concerned about any particular knowledge since it usually takes part of the first year of a PhD to acquire the background required to begin carrying out research. A good start would be the MSc in Mathematical Logic and the Theory of Computation on which I teach.
A general introduction into pursuing a PhD at the School of Computer Science is available here. Specific enquiries may also be directed to me personally.