Like all of your passive and active fire safety equipment within your building, it should be maintained by a competent specialist who holds the appropriate insurances and UKAS accreditation. This means their practices are audited by third-party certification schemes and follow British Standards.
Always remember to ask for accreditation to ensure the provider you choose is trustworthy and competent to service your building’s fire doors, and retain the service reports for your records as evidence of your due diligence to keep your building safe and legal.