It seems that it might be the switch or sensor that tells the motor to take up the tension- the "door open" light on the dash is constantly on, and the boot courtesy light stays on as well as though the boot isn't shut properly. I may end up taking it to an auto electrician to have a look at the switch and sensor, a better option than having water leak into the boot I think.
I haven't given it a really hard slam but a firm push doesn't seem to help, it latches with a gentle push, just wont take up the slack like it used to. The joys of an electric boot latch!