The HSE regulations make a distinction between gas safety checks and the related servicing and maintenance activities that may need to be undertaken. As a landlord you have the responsibility of ensuring all appliances (specifically boilers), as well as pipes and flues, are operating safely at all times.
Our recommendation is that maintenance be performed every 12 months to ensure proper functioning of the appliances.
For full guidance refer to regulation 36(2) of the HSE Approved Code of Practice, on the duties of landlords with respect to maintaining pipes, flues and appliances.