We've seen this question before and I'm pretty sure there is not even a third party effect available that simulates lightning. IIRC, the advice was to locate a movie and either buy it or find a freebie.
If you know someone with After Effects, there are two fabulous lightning simulations. However, this effect is supremely fake. Real lightning is instantaneous and simulations tend to last much longer, instantly flagging the visual as faked.
Thanks very much for confirming that David, I guess AE is they way to go then...I don't mind a bit of fake looking lightning, it's for an 8mm promo anyway so it doesn't have to be totally realistic, in fact I need it to look almost cartoon like.