I've got no idea of what the statistics are re: applications per place, but I suspect it isn't as high as, say, the full-time Computer Science MSc because:
1. It's a part-time course. Full-time courses have limited capacity because of the requirement for each student to be a member of a college. On the Software Engineering programme, you don't become a member of a college until you transfer from the Postgrad Certificate to the MSc. Even then, most students end up at Kellogg which is catered for part-time students.
2. It's an expensive course. You pay somewhere in the region of 17K over a 3 or so year period. This makes the programme somewhat more expensive than the 1 year, full-time Comp Sci MSc.