If you’ve got a roadster like me, have you checked that the rear drip trays for the roof drainage aren’t blocked? - they’re located under each of the roof flap servo covers that raise and lower on roof operation - if you open the roof half way, you should able to peer down into the mechanism to check visually……if you see standing water in the trays, likely they are blocked.

In my experience, any water that overflows the trays falls down and can make its way past the rear bulkhead behind the seats and into the boot area where it can slosh about and typically ends up in and around the battery as it’s the lowest point.

I‘d check this first to rule it out.

 

Thanks Stevie, going to look there first. Getting some curtain rod or strimmer wire together to attempt a drain clean. So I'm guessing the water comes in as you describe, within that 'skin' (why would they not have made it one solid base?) then sloshes up. Since the boot floor appears bone dry at the rear...