Here is the wording from the 2013 Home Office Firearms Guidance on Firearms Law:
Borrowed rifles on private premises
Section 16(1) of the 1988 Act enables a person over the age of seventeen to borrow a riflefrom the occupier of private premises and to use it on those premises in the presence ofeither the occupier or their servant without holding a firearm certificate in respect of thatrifle. It should be noted that this gives slightly more flexibility in the use of a borrowed riflethan is permissible with the use of a shotgun as described in paragraph 6.18, in that theborrowed rifle can also be used in the presence of the servant of the occupier.
The occupier and/or their servant must hold a firearm certificate in respect of the firearmbeing 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 thecertificate. These may include a safekeeping requirement and, in some cases, territorialrestrictions. It is also a requirement where the borrower is seventeen that the occupieror servant is eighteen or over – this was inserted by SI 2010/1759. Section 57(4) of the1968 Act defines "premises" as including any land. The effect of the provision is to allow aperson visiting a private land to borrow and use a rifle without a certificate. The exemptiondoes not extend to persons under the age of seventeen or to other types of firearm. Thereis no notification required on the loan of a firearm under these circumstances. A borrowedrifle should not be specifically identified as such on a "keeper’s" or "landowner’s" firearmcertificate. The term "in the presence of" is not defined in law but is generally interpreted asbeing within sight or earshot.
Section 16(2) of the 1988 Act provides for a person borrowing a rifle in accordance withsection 16(1) of the 1988 Act to purchase or acquire ammunition for use in the rifle, and tohave it in their possession during the period for which the rifle is borrowed, without holdinga certificate. The borrower’s possession of the ammunition must comply with the conditionson the certificate of the person in whose presence they are and the amount of ammunitionborrowed must not exceed that which the certificate holder is authorised to have in theirpossession at that time. It should be noted that the borrower may only take possessionof the ammunition during the period of the loan of the rifle at which time they will be in thepresence of the certificate holder. If the persons selling or handing over the ammunitionare not certificate holders, it may be necessary for them to see the certificate to satisfythemselves that the terms of this section have been met and that the amount of ammunitionthe borrower wishes to acquire is no greater than that which the certificate holder isauthorised to possess. The details of the transaction need not be recorded on the certificate.