Bradley

Hello. I wonder if anyone can help. I am carrying out alarm service and testing in various HMO. Aditioanlly each year we have to test the flats mains detector in hallway linked to fire alalrm panel. I am having difficaulty in access to these properties. I send a 1 letter out a month before inspection with certain day for test. this is then followed up by a reminder letter 2 week before service. All with contact number if resident wishes to change date/time etc. I then attend and get no access. I card them, but still get no reply. My question is. Is there a reasonable protocol or process / guideline to state How many times we try to gain access and if detector can be noted as not accessed and rolled over to next year. Looked everywhere but cant find any guidelines Thanks Bradley