On 07/08/2015 12:11, Bill wrote:

Snip.

Its probably a teenage son.

Any thoughts welcome on what else to think about and what to suggest
they say, politely, in response.

They aren't claiming they downloaded the film, they are claiming it was
made available for others to download.
Of course torrent clients do this if you let them.
It is this that is illegal.

Maybe you could claim they were not authorised to use the computer to
download anything and claim they were breaking the computer misuse act?
They couldn't know it was there without them trying to download it.

Or you could just tell them you have no idea what they are talking about
and to send you the evidence with an explanation as to what it means.

Or just ignore them which is what I would do, after all they can't prove
you have ever seen the letter.