Ok I gave it a try this morning -(chickened out and used the subway yesterday - didn't think it was the best time to experiment in the rain).
I've been having a look at cycle craft aswell since I got it on Monday, and the conclusion I got to from both reading that and the advice from here is that Primary all the way is best.
So this morning I just sat square behind the car in front of me (about 6 cars back from the lights), dropped a couple of gears to get a faster acceleration and went for it when the lights changed, I didn't try to filter to the front as I thought that jumping the queue then sitting in front would be taking the P'. Luckily it was a bit slow on the other side of the junction so the car in front didn't get too far ahead and I ended up having to wait behind him, then pedal again like crazy, staying in Primary all the way until my left turn about 100 metres on (at a guess) to stop any dodgy overtakes. The WVM behind me got a little impatient towards the end and revved his engine a little bit - probably thinking I was going to block him all the way, he stopped as soon as I signalled left though. Was pooped by the time I got round the corner though - talk about a work out.
So - so far, so good.
Will give it a few more tries and see how it goes.
Thanks for the tips guys.