I'd like to replace the water main to the house. The water company
have
asked for a trench 750mm deep lined with pea-gravel or similar do be
dug
from teh house to the road.

The biggest problem is that the trench will have to go through/under
the
garden wall which is a 120 year old dry stone construction which may
well not cope well.

However , when a gas supply was installed, Transo used some sort of
tunneling device to connect two small holes. I wasn't at home at the
time so didn't see the operation, but a now wondering if a similar
machine could be used to bore a tunnel for the water pipe.

Can these machines be hired or does one have to get a specialist in?

One can hire them with an operator but they ain't cheap. For a 40 metre
long bore including access works I was quoted £25k.