can't find my reference but from memory it is 12 months.

The HSE guidance: The manufacturer's instructions should describe what maintenance is required to keep the equipment safe and how this can be done safely. These instructions should always be followed, unless there are justifiable reasons for not doing so (eg where more frequent maintenance is necessary, due to intense use, adverse environmental conditions or when other experience shows this need). Maintenance on a less frequent basis than the manufacturer's recommendation should be subject to careful risk assessment and the reasons for doing so should be reviewed at appropriate intervals. For example, where there is already an inspection regime, perhaps for lightly used equipment, less frequent maintenance may be justified because of the condition monitoring already provided by the inspection programme.

If there is no manufacturer info available you should arrange an inspection by a competent person to decide what is required going forward based on condition, arrangement, use and so on.