Home insurance:
Musical instruments are covered under Contents for the following:

Within the home - Cover limits vary between policies so please refer to your policy booklet for the relevant cover limit.

Stored in an outbuilding - Cover limits vary between policies so please refer to your policy booklet for the relevant cover limit.

Cover for Musical Instruments is excluded in the Personal Possessions section of the policy.

Tenants contents insurance:
Musical instruments are covered under Contents for the following:

Stored in an outbuilding – Maximum £3,000 for theft or sums insured for all other risks.
Cover for Musical Instruments is excluded in the Worldwide Belongings section of the policy.