There would seem to be two possibilities, firstly it just needs adjusting.

To do this you need to remove the undertrays.

This can be done in the middle only and on one side and you can get access

(use plus gas to free the bolts as they have a tendancy to snap).

With the handbrake off the adjusting mechanism will be very clear.

Secondly, if the handbrake is very bad have you had the rear brake pads replaced? They have to be done in a special way as otherwise you get a crap or no handbrake.

There is an internal mechaism in the rear calipers and this has to be re-set otherwise as above.

The pistons need to be screwed inwards to the full extent and then unscrewed 180 degrees, this re-sets the handbrake mechanism.

Frequently this is fiddly as the piston screws out then you can't get the pads in.

I had to go through this process about 4 times until it worked.

Hope this helps

Geoff