hi Sarah,
When i purchased my GQ i had very similar problem. MY Starter would only click when in the car, so i ordered a new starter and replaced the old one. The problem was the old starter was the wrong one. The person who owned the car before me had put the wrong starter in.(i think they purchased it from a wreckers) Therefore when my solenoid when to engage the shaft, the shaft of the starter motor was already engaged onto the fly wheel. When i removed the old starter the flywheel teeth were a little worn. So this no doubt will have repercussions later.
I only noticed it, when i put both starter motors side by side on the ground to work out the difference. With the new starter the ol patrol starts perfectly.