What the Law Requires
The Fire Safety Order 2005 requires that Fire Fighting apparatus including fire extinguishers receive regular testing and servicing.
Further detail is provided by British Standard 5306 Part 3; 2015, and generally, business owners should ensure that their extinguishers are checked by suitably competent engineers annually.
So what's a 'competent engineer'?
Well one who has qualified to BAFE (or similar) standards.
All Unicorn Extinguisher Technicians are fully qualified.