Can you please advise on the optimal position for my atu. I'm a novice ham,
and at the moment I've got a random length of wire slung between my attic
roof and a garden tree (about 40 feet high), with a connection to the centre
wire of a 50 ohm coax cable as soon as it enters the attic roof, down to my
antenna tuner.

From what you have written, I surmise that you do not have
a ground connection for the coax shield at the antenna
feedpoint, that is, at the attic roof. I would add such
a wire from that point down to a ground rod outside your
house. Attaching some buried ground wires to that rod
would help even more.

You might also add a balun between the coax line and the
antenna feedpoint. This can be simply another length of
coax coiled up at the feedpoint and connected between the
existing coax and the antenna. You didn't mention the
frequencies you are using the antenna on. For lower
frequencies, you would need more coax. Assuming a lower
frequency of 7 MHz, a 20-foot length of coax formed into
a 12-inch diameter coil should suffice. Commercial baluns
are relatively cheap and small. If you use one of those,
get a 1:1 type and make sure you connect the ground wire
to the wire on the balun that has a DC connection to
the coax connector shield. Or get a special form of the
balun called an UNUN and connect the ground wire to the
shield of its isolated coax connector and the antenna
to the center pin.

The antenna you have constructed, although having a
horizontal wire, actually responds to signals that
are vertically polarized, particularly at the lower
frequencies. Unfortunately, noise is also predominantly
vertically polarized, so the antenna is somewhat inherently
noisy.

Of course, I assume you have already followed the mainline
advice offered to you by other posters here that you
eliminate, reduce the effect of, or at least identify the
noisy appliances in your own house.

Would it be any better if the atu was between
the wire and the coax where it enters the roof (although it would be
difficult for me to get a power supply up there)?

I wouldn't do that unless you were having trouble getting
an impedance match on the higher frequencies. And to make
the ATU work at the feedpoint, a ground connection from
there is absolutely required.

Hope this helps....

Jim Bromley, K7JEB
Glendale, AZ, USA