This has just been discussed over on the Yahoo Omega group - it appears that lots of people have this problem - the solution proposed was (copied):
The procedure is open bonnet get some adjustable grips that fit around the base of the striking pin and undo slightly get screw driver in the pin itself and wind it in half a turn then tighten up the base again trying not to move it out of position then shut bonnet if need be do it again until she opens nice
There appears to be a slight diffrence between pre and post facelifts, see following:
I think there may be a differance between pre and post facelift
bonnets. I have also had the problem of the bonnet release catch
having to be pulled to it's maximum to release the bonnet and even
after cleaning, lubricating all moving parts with 3 in 1 ( don't use
WD 40 ) and then smearing coppa slip and grease around the pin it
still would " stick " After reading these posts I went out this
evening to adjust it as suggested. I found that the spring pin on my
facelift model had two flats on the begining of the taper of the pin
but no slots.Using a 19 mm spanner I wound the cone 1/2 a turn
clockwise as implied, shut it and spent the next hour trying to
release the bonnet.Once opened I then wound it anticlockwise 1 turn (
1/2 turn ac from original position ) and this made a big
differance.It now releases first time everytime.
Hope this helps cause I wouldn't want to have to free that bloody
thing in the dark and rain .....