Hi there
I think that depending on which university you apply to, recruitment for masters courses takes place throughout the year. When I applied for my masters course in English Literature I was told it would be pretty competitive and to apply as early as possible. I also wanted to apply for AHRC funding, so I knew I had to have an offer of a place by at least January, as I needed to get my AHRC form to my referees etc by Feb/March, so I applied for my masters in October 07 (for Sept 08 start) and got offered my place in December. This was very early though, and I think that you should be okay if you apply at the beginning of April - the best thing you can do is email the postgraduate admissions team at your chosen university and ask them.