I'm in the same boat Rach - got a 13 year old who smokes.
This has been going on for around a year with Kayleigh, and we have tried everything. We have spoken to her in depth about it, rationalised it, ignored it, punished her for it, screamed at her for it, and everything, but nothing has changed.
It's an awful thing to say, but I can remember being that age (just!) and if they want to do it, they will no matter what you say.
I actually thought we'd cracked it a couple of weeks ago. She hasn't had pocket money for a year now, and has to take sandwiches to school, she is never ever given money as we don't want her to be tempted. So we hadn't had anything for a few weeks, and I was actually talking about beginning to give her pocket money again in a week or so. Then on Saturday, DH spotted her throw something into the neighbours bush on the way home. He went to investigate and it was a lighter. I asked to smell her fingers, and she had been smoking. So now she is grounded again, not allowed to even phone her friends, and no money for the foreseeable future.
Urchin - I have asked Kayleigh on many occasions where she gets the money from, and her answer if that 'friends' give them to her, or they go '2's up' on a ciggie. I cannot ban her from seeing her friends at school, and can't dictate to her who she hangs around with. One of her friends is 16 and her mum gives her money instead of affection, and she buys fags for everyone. I've even reported the shop to the police for selling to under 16's but nothing was done.
I have explained to her how hard it is to quit once you are addicted, and also told her how it is becoming less sociably accepted, but nothing is sinking in. She understands completely that the law is changing next summer, so nobody will even be allowed to smoke in a pub, and it will be even stricter when she gets older, but it's not changing anything.
The infuriating thing is that she is extremely inteligent!! She is in the gifted and learning acadamy at school and is taking her French GCSE next June at the age of 14. But still she does this.
I know I've not been much help at all to you Rach, but I wanted you to see it isn't anything you have done. Kids will try these things, and there isn't a lot you can do to stop them. Just be there to let them talk if they want to.