Home Office say that you need to be within ear shot/sight, now it doesn't say if that's for the shot to be fired or being able to talk to him or viewing through Binos.........Radios would make for a very long "Hearing Distance".
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http://police.homeoffice.gov.uk/pub...policing/HO-Firearms-Guidance.pdf?view=Binary
Borrowed rifles on private premises
6.16 Section 16(1) of the 1988 Act enables a
person to borrow a rifle from the occupier
of private premises and to use it on those
premises in the presence of either the
occupier or their servant without holding a
firearm certificate in respect of that rifle. It
should be noted that this gives slightly more
flexibility in the use of a borrowed rifle than
is permissible with the use of a shot gun
as described in paragraph 6.14, in that the
borrowed rifle can also be used in the
presence of the servant of the occupier.
However, the occupier and/or their servant
must hold a firearm certificate in respect of
the firearm being used, and the borrower,
who must be accompanied by the certificate
holder (whether it is the occupier or their
servant), must comply with the conditions
of the certificate. These may include a
safekeeping requirement and, in some cases,
territorial restrictions. Section 57(4) of the
1968 Act defines “premises” as including any
land. The effect of the provision is to allow
a person visiting a private estate to borrow
and use a rifle without a certificate. The
exemption does not extend to persons under
the age of 17 or to other types of firearm.
There is no notification required on the
loan of a firearm under these circumstances.
A borrowed rifle should not be specifically
identified as such on a “keeper’s” or
“landowner’s” firearm certificate. The term
“in the presence of” is not defined in law
but is generally interpreted as being within
sight and earshot.