Is it only items of equipment supplied via a 13amp plug that require to be tested during combined inspection and test of electrical appliances?

Answer:

The Electricity at Work Regulations 1989 (Regulation 4) require all electrical equipment in a place of work to be maintained so as to prevent danger. It is therefore important that all equipment is inspected and tested.
The frequency of such testing is of course dependant on its method of use, and portable and hand held equipment would require more frequent attention than stationary equipment.

The IEE Code of Practice for In-Service Inspection and Testing of Electrical Equipment gives guidance on this subject.