Ok this is just getting annoying... every time i drive up and down my road, which is a bit bumpy, the bonnet (front) of my '72 Notchback pops up.. the latch seems fine, and when you push it down closed you can't pull it back up no matter how hard you try..

So yeah... any ideas and is this a common problem?

This exact thing has happened to me, thank god for the safety catch. It is could be a couple of things:
Is the release cable from the glove box really tight? If so it could be pulling the latch and opening it.

As a precaution remove the spare tyre and the spare tyre well, so you can access the latch from underneath. This way you can'nt lock yourself while adjusting the catch. Undo the lock nut on the rod and wind it out a couple of turns, tighten up the lock nut and drive up and down the driveway to test. Also have a good look at the latch on the bonnet lid and the catch checking for any signs or wear. Check the return spring under the latch. You will need to stick your head in the bonnet and look up or lie under the car and look up through the hole where the spare tyre was. Sometimes the spring does not hold the catch in place firmly enough allowing it to open when bounced.

Yes, sounds like the cable is too tight... with pressure on the latch all the time... very unusual... so the latch doesn't actually go right into the groove.... as far as it should...

try pushing down then pulling up and see if it opens....
if You can= I just remembered that type 3s didn't have a pull handle...
undo the locknut and screw the plunger 1/2 a turn anticlockwise.. tighten locknut - not too tight and see if it still pops up... just keep doing that until it stays shut...
If the plunger is worn in one spot then 1/2 a turn should fix it.. but anything is possible...
The plunger should be lubricated with petroleum jelly or similar... if You use grease and it gets on Your clothes...
well lithium based grease stays in clothes.... forever..

remember everything that moves should be lubricated...
including door hinges locks wiper spindle shafts etc etc...
all these used to be done at every 3,000 mile service in the old days by the mechanic.... that is why they have lasted so long... make sure You keep doing it...

Lee

Thanks guys, i'll try out your advice...