Mark is one of the most contactable writer/comedians in the industry. He has a MySpace page and a Facebook Fan Page which you can sign up to in order to be kept abreast of virtually every movement of his waking life. He quite often replies to things which other comedians would haughtily ignore.
For professional inquiries, though, a much more reliable route is to contact one of the following people:
Christian Knowles at CKP
Tel 020 317 66971