Maternity covers are great because you do the full job for two terms, so can get a better reference (you can be a form tutor, do assessments, deal with parents, contribute to a team, etc), and there is a good chance of the job becomming permanent. They are hard to come by as schools often like maternity covers to hit the ground running, especially if it starts mid year.
My friend went for a interview for a maternity cover a few weeks ago, and there were over 30 applicants, according to the school.
My advice to the OP would be to sign up with a teaching agency in order to get a CRB check. The agency will need two referees, which can be the tricky part. I used my DSs' head teacher, who was a personal friend and my vicar. They could only give character references, of course, and that was fine. I'm not sure if it is still necessary to sign up with the GTC, but if so, do this plus a union. Then you will be ready for any opportunity that comes your way.
Also, create a CV, and try to present your time off in the most positive light, thinking about the transferrable skills you acquired in that time. You will have some difficult questions to answer, so be prepared for these.
And, to show that you are investing in your career, ask some schools to let you shadow them for a week or two at a time. Most schools will let you do this, and you will be on their radar screen for when they are in need of a supply teacher. You can also pick the head teachers' brains as to how to market yourself effectively.