Would someone tell me about supply teaching (secondary)

[EdwiniasRevenge]

I've just finished my PGCE (secondary science). Its been a long slog as I took 18m out due to ill health. I have been applying for jobs and can get interviews but no luck with the job. So I think I'm going to have to go down the supply route to get a foot in the door.

what should I look for in a supply agency? What will the work be like (I.e. how will it be different to FT perm position)? How many agencies should I sign up with?

You have to be organised and resilient - ask around agencies before you sign up to any. If they insist you use an umbrella company for your pay, don't touch them. The reputable ones will pay you PAYE. You will be tempted by the offer or reclaiming your mileage etc to join an umbrella agency, but in fact you can reclaim this anyway though your tax return, and can save on the cost that YOU will have to pay the umbrella company.
Agencies prefer the umbrella arrangement because they save on employers' NI - tax avoidance by any other name.