Found this on the site governing trading standards re: selling knives:
To avoid committing these offences, it is advised that the legislation is brought to the attention of all staff via regular training. It is important that you can prove that your staff have understood what is required of them under the legislation. This can be done by keeping a record of the training and asking the member of staff to sign to say that they have understood it. These records should then be checked and signed on a regular basis by management or the owner.
Knives should be displayed where staff can see them.
Last sentence is interesting.