Thanks for the response so far.

Yes I did mean that the tiolets had no external overflow.

With regards to the situation of the overflow pipe, it's not obvious to
trace it back (well to me anyway). It's a town house and the overflow
pipe is on the side wall about a foot from the ground. The hot water
tank is on the first floor.

If it is condensation from the hot water tank, what can I do about it?

Thanks again.