i'm wanting to put my AP 330 kit on the back of my escos so i can put 6 pot 355's on the front, my problem being what do I do about the handbrake? i have heard that hydraulic handbrakes release pressure when the car is stationary and that the car can roll??? is this bollox or true as i know they are illegal for road use but i can get round that.
Is 100% true, however the simple solution is if parking on uneven ground, put the car in-gear as well as handbrake. If you use just a hydraulic handbrake - don't expect to find the car where you left it (hence why it is an MOT failure).

I've also heard about a seperate handbrake caliper similar to an RS200 one you can fit for the handbrake cable, i think turbosystems done this to an escos a while ago??
This is also correct, but I think you will now struggle to readily find this set-up. I have a hydraulic set-up like you mention (315s on the back / 378 on front) and just ensure that the car is in gear as well when parked anywhere. I did not want the extra weight of a mechanical set-up.

Also, finally, would i need to fit a bias valve or something because i'm fitting big brakes on the back?
Absolutely 100%, as you can set the car up how you want it to brake (which is different wet / dry) to get the absolute best balance you can for the relevant conditions..