@KB The NJ templates have it pretty much covered I reckon but this might be helpful too, just in general (don't bother about the title page though).
http://downloads.bbc.co.uk/writersroom/scripts/bbcradiocue.pdf
This may be sucking on obvious eggs, but remember to title your file attachment with your name and sketch title so if (and that's a massive gargantuan IF in my case) they like it, they can find it again without going round the houses.
Right, back to laughing at my own oneliners!
Good luck with it, hope you hit a home run