Sorted! Much easier than thought - by removing the lower clamp bracket spacer and bolts (see attached pic - parts 8 & 9 - this pic should have been in the OP) you can simply swivel each half of the main clamp and this allows the pin to be removed by pushing one way then the other. No need to touch the bracket swivel pin or main tilt tube and loctited nuts.
I straightened the pin (to temporarily try it - will replace with a new one) and it all works perfectly now.
Only slight issue with this system is that to tilt the outboard to the fully up locked position you don't pull it all the way up but one click or so from the top. It then settles into the 'stay up' position. If you pull it further up the mechanism moves it into the back down position.
Also for shallow running you can lift it up a notch or more but to get it back down you need to lift it right up again - normally when approaching shallow water I leave it running, lift up then drop it down - with the mariner to get it back fully down you cannot just 'pull a pin' and lower it back, you need to pull the whole engine up out of the water, not a good idea with it running....
How do others get on with this?