????? really ? ?????
Im with cookson, clearly the handbrake isnt holding , so either the cables need adjusting slightly or there is a fault, common for snapping handbrake cables, also if your pads aren't great this maybe the reason!
The clutch is to drive the wheels so when its down it wont have any effect unless its dragging , but that would pull you forward !
Have you tried it not in gear ? can guarantee itll be the same ! hence eliminating the the drivetrain