Actually, it's spelled "mattins." In the Church of England, it's the morning prayer service. On Sundays, the service is at 10:15 AM, according to the current worship schedule for St. Paul's:
http://www.stpauls.co.uk/documents/musi ... er%20i.pdf
I have also been to the Sung Eucharist service there. Which service I attend is mostly dependent on how early I can convince myself to get over to the church.
Speaking of services, I spent Christmas in London a number of years ago. I attended the midnight Christmas Eve service at St. Paul's, and the Christmas Day Evensong service at Westminster Abbey. At the Evensong service, I sat on Charles Dickens. I am serious. Folding chairs were set up in the Poet's Corner, and my chair was located on Dickens' tomb in the floor.
If you're ever in London over Christmas, I recommend doing both services. But be sure that you have pre-arranged transportation back to your hotel after the midnight service, because the tube quits running at midnight, and it is very difficult to catch a cab after the service, as my friend and I found out.