Looking through the grill i can see a hook which i can push aside with a screwdriver. Also when cable is pulled i can see a "bar" going up and down but the bonnet won't pop open. Any ideas? Thanks

 Might be stuck down with tree sap...

Get someone to pull the lever while you pull the bonnet up or even push it down and it might pop up.

If not, the cable may have snapped inside the mechanism point or mechanism is broke... but unfortunatly, there is a protector around the actualy mechanism to prevent theives from teevin you engine (well.....its like that on my mums old bmw so assuming its the same.)

To open the bonnet, you have to pull a little rubber bit that pops out. try see what that attches to and yank that with some plies.

When you pull the lever inside the car, does the bonnet pop up at all ?

If this is the case, there's not a lot you can do

 I'd go the pushing down bit rather than trying to yank it up. If/when you open it try adjusting the rubber stops to get abit more lift when the cable is pulled - and lube the mechanism before you drop it again.

Have you tried prayer?

Failing that, a mechanic?

mine did this when i first got it, the bonnet wouldnt open because it had seized up as it had been sat, all the mechanic did was spray the latch (through the gap between the bonnet and bumper) with WD40, then push down and pull up whilst some one is pulling the release switch in the drivers footwell hope this helps

 Thanks for your ideas guys. I've soaked it in WD40 and will try again in the morning before it warms up. There is something moving when the lever is pulled put bonnet not popping at all?  

 SOLUTION

On the back of the locking part is a plastic guide which guides the cable to the part it pulls. This had come out of its mountings and the cable was to slack to pull the lever. If you can reach through the grill and find the cable(i probably not) pull it towards the passenger side. If you can get enough pull it should open. I think the German cars probably run cable from this(their drivers side). The plastic bit that came off on mine guides it from our drivers side past centre to that side so it is pulling back on itself.

If you can't grab cable like me you will have to dismantle. Remove VW badge(twist and pull) 2 torx behind here and 1 more torx about 2cm to the left of the one on the left. For this on i had to drill my GTI grill to get to it(doesn't show) other grills may be easier. You can then lift out the whole locking mech and see what your doing.

Rob you read my mind, I had to do the exact same thing to mine I changed the front end and forgot to slip that bit of cable back onto where it goes so once the bonnet closed it was not opening again.  I had to do what Rob suggested and take the bolts out and lift the bonnet up with the mechanism attached.

Darren