If you wish to conform to BS 5839 part 6, it states in relations to grade C systems,
"All systems, other than Grade A systems, should be tested at least every week by operating all fire alarm devices in the dwelling, Grade B and Grade C systems should be serviced every six months in accordance with the supplier?s instructions. Where experience shows that undue deposits of dust or dirt are likely to accumulate, so affecting the performance of the system before detectors are cleaned or changed at the recommended service, more frequent cleaning or changing of detectors should be carried out."
To me this would this reads as the system should be tested every week and serviced every six months. The heads should be cleaned at the six month service and more frequent if necessary. Therefore what has been suggested does not conform to BS 5839 part 6.