I don't understand whether you're going for a Masters or a PhD since you haven't specified and unless you're going for an MRes..? But..
I think email is just best for time reasons, they can read it at their leisure and it means you don't have to make time during office hours during which they might not even be available. It also means you can get across what background you're coming from with a little more grace (I don't know about you but I get nervous on calls to superiors!) and intelligence and you can really think through what you want to say.
Mention where and when you met, but don't expect him to remember you since they're busy and network a lot (understandable), then I would go on to say that you're contacting him since he advised you to, say you'd like to meet up to discuss the possibility of doing a masters/PhD. Suggest a date within a few weeks time, then mention your availability in cse that date doesn't suit him.