Do you have the type where you have to put the key in behind the badge or the type with the release inside the car?
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It is most likely a seized mechanism.
you will need some wd40 or somthing similar, spray the whole mechanism, then locate where the cable attaches to the mechanism, lever this back and forth with a screwdriver, keep spraying it as you work it and it will eventually become free and move back against the spring on its own.
You will probably not need a trip to the garage, these seize up regularly because of all the salt and muck on the road getting into them
I hope this helps, if you need more info, please let me know.
Hmmm, that hasn't worked.
It doesn't seem to be seized, it seems that the secondary part of the clip isn't catching, just springing back.
The part that springs back should be held closed by a small secondary clip, this is the oposite side to the cable, it is operated by the cable, so it is either seized, or the cable is seized.
The catch seems to be catching now when you press it with a screwdriver, it just wont catch when you close the bonnet. Its like the bracket isn't going far enough into the catch.
Unfortunately its getting later and neighbours have small children so I am going to have to stop banging the bonnet down till the morning
check that the part of the slap panel where the catch sits is not bent down, also check that there is nothing caught in the bonnet itself.