The button had stopped working so I found you guys on this forum (wish i'd found you before...soo much info!) and after watching the manual roof opening process I did one successfully and was on the second when the trunk locked.
I now strongly believe it's the pump that will be sitting in water, the poor old girl has sat outside unused as of our first lockdown.
After watching the trunk opening video I jumped to the task, only to find the anti-theft plates either side of the latch that stops me manually opening it.
I have a camera probe so will take some video of the situation and as much of the internals as I can find with it and pop it up here...once it stops snowing!

Thank you for your help, I really appreciate it.