Hi Matt,
It the catch isn't holding then there is obviously a reson for this.
Try checking the release cable, if it is to tight then the latch jaws will not be closed properly. Check the male part of the latch i.e. the part attached to the bonnet. this has some adjustment which will be obvious if the bolts are loose then it may be out of alignment. could also be worn.
I would go round ever nut and bolt on the bonnet including the hinges and makesure then are tight enough. If there is slack at the bolts the bonnet will vibrate and that will also cause the bonnet to release.
I have had similar problems in the past and also the opposite problem of not releasing. I cured the problems by loosening the latch then with a bit of trial and error moving back, forward, left, right until I found the best position. It wont really matter whether you do this with the male or female part of the latch but don't recommend having both of them slack at the same time.
Once you have found the best adjustment close bonnet and check that it releases ok and then try pull it open. You will practically be able to lift the front end of the car.
colin