Renault Megan coupe 2001 - boot wont lock?

I have a Renault Megan Coupe ( April 2001) The boot will no longer lock, the rest of the central locking works perfectly. The locking mechanism uses a small motor to move the latch mechanism away from the boot release button.

Motor & mechanism are free and move easily, there are only 2 wires connected to the motor - both go back into the wiring loom. I assume that as the motor is DC and it has to move in both directions the polarity change will be controlled from a central point, wherever that is.

Any ideas before Renault tell me I need a very expensive part