How do I get a foot in the door of secondary education teaching?

[nodplod]

Are there any teachers out there (UK) who could give me some tips for applying to secondary schools for TA/Cover supervisor jobs?
I've been applying for teaching/learning support assistant posts, cover supervisor posts, and even volunteer work in secondary schools for the last three years. I teach ESOL to adults but also keep up to date with the National curriculum by tutoring secondary pupils in English and Spanish. I have a Diploma in Spanish, CELTA and have almost completed my qualification to teach post 16; however I have always wanted to work with the 11-18 age group, but as I am a single parent I have had to remain in my current employment to survive.
What do I have to include on my cover letters to get an interview at least? I am articulate, keen, enthusiastic, hardworking and have good references. Even with jobs related to students with ESOL needs I just don't get as far as interview. A friend suggested I may be overqualified but what more can I do to convince them? Ideas?
Sorry I meant teaching assistant-I haven't applied for any teaching jobs as I am aware of the quals required. It's just that I know TA posts do not require PGCE and some of the TAs I have encountered in the sector though my children's education seem to be virtually unqualified!!