Yes. If you have more than 5 staff on the premises, it is a legal requirement.
No. It is the responsibility of the company to make sure an annual service is carried out on the building/premises each and every year.
All fire extinguishers should be commissioned into service by a qualified engineer and serviced annually in accordance with British Standard EN3 2005. This is the British Standard that insurance companies use as a rule of thumb. Without a current Certificate of Maintenance your policy is void but some insurance companies may not make this information clear to their policyholders.
This is most likely covering his/her building and not your contents within the unit - that comes under your own business insurance. Unless your (written) agreement states otherwise, it is unlikely that in the case of a fire any contents in the unit would be insured. Any claim would be through your own business insurance and not your landlord. Relying on a landlord's insurance is a common and costly mistake - one which may only be discovered after a claim is submitted.
From 10-20 years depending on the type.
We only require 4 weeks written notice.