Thanks to those who explained about the 300mA ELCB. I've been and had a
look. The ELCB does trip when you press the test button. I don't know
why she said it didn't. Anyway, here's a picture of the consumer unit:
https://dl.dropbox.com/u/11023364/201211121665.jpg

There is no earth coming in with the supply, just a large flat cable
containing line and neutral. The only earth is a 4mm yellow/green cable
running to an earth rod in the garden.

What can be done to make the installation safe? It's a very small house.
One of the breakers is disused, because they don't have an immersion any
more.

Thanks in advance. I'll like to help her get this sorted out economically.

Bill