There's a discrepency between the BJ website and the TLI one; according to the latter it's..
20th June
4th July
18th July
1st August
As Colin said, you can ride all four if you get someone to marshall for you, but you don't get any more series points (in fact unless you win, you get less!)
Excellent. I'll be marshalling the first one as I'm doing a RR over near Huddersfield the following evening. So I'll get 50 points and be joint series leader for a fortnight. Yay!!!