On inspection, I found water under the rear seats and in the boot.
I have tested the light clusters and no water ingress there.
I pealed the roof lining back to reveal the sun roof drain, as in Jim's blog. And found all to be OK.
I found the seal on the rear windscreen to be slightly perished in a couple of places. Would this let water into the boot? Or would it be channeled out somewhere?
On inspection, it looks like water may get in past the rubber buffers on the underside of the hatch, if water gets past the screen seal.
Hose pipe test is looming.