To finish,
I filled the hole (with that messy expanding foam stuff) but still the water got in so I took the Old Girl around to the panel shop. There was a few rust holes as well as that bolt hole, but also a hole up under the chrome windscreen trim. The windscreen hasn't been sealed and water tends to bead in the corners, so I've got some mastic for that...
The driver's door appears to be sitting low - it's not sealing against the rubbers at the top and letting in water as well. I'll try loosening the door/hinge mounts and lifting it a bit (they're not countersunk like the hinge/body mounts are they?
)
It's funny how you find these faults with your bus only after some decent rain!
Thanks to everyone again...
Pete