Recently had my plumber round to connect up the overflow
pipe for the boiler he installed last year (the pipe
needed to go through the kitchen, and I was planning
on installing a new one, which I now am), so in the
meantime he left it running through the bathroom
overflow.

He's drilled a hole in the outside wall, and the overflow
terminates in a bit of 15mm copper sticking out of that.
It seems to me that I ought to seal the hole with something
- but what?

Ben