Lift the bonnet, there is a black plastic cover over the radiator which is in 3 sections, remove the right hand side section near the battery, you will see the bonnet release cable, the outer section of the cable is held inplace with a small bolt (think its 8mm) undo the bolt and pull the outer cable towards the wing and retighten the bolt. I would only pull the cable in small amounts and keep testing, you don't want to over do it and have the bonnet open while your driving 
Took some pictures whilst revisiting mine today:
First, remove the plastic cover by the battery as mentioned by Elite Pete - to do this, turn the large screw 1/4 turn, and gently pull the panel off whilst moving it towards the engine (there is a plastic lug at the back, and if you pull it straight up it can snap as it did with me before):

Once the cover is off, you can see the release cable coming around from the wing. The bolt you need to undo is the rusty looking one holding the cable at the bottom left of the picture....

Before you do it, make sure the release lever under the dash is fully home. Carefully loosen the bolt, so that the cable is free (you may need to give it a spray with WD40 or similar if it has never been adjusted before) - it will indeed require an 8mm socket. I would then pull the cable only about 5mm towards the wing of the car, then nip the bolt back up. Check the cable is still operating the release, by pulling the bonnet lever, then shut the bonnet and give it a go. If there is still a lot of travel, open the bonnet again, loosen the bolt again, and move the cable another few mm and repeat. Here is a better picture of the bolt:

When you are happy, fully tighten the bolt and replace the plastic cover, and you are good to go
