"The recommended period between successive inspection and
servicing visits should not exceed six months. If this recommendation is not implemented, it should be
considered that the system is no longer compliant with this part of BS 5839."
"The work described may be carried out over the course of two or more service visits during each twelve month period."
Whilst much is duplicated, once per year every device must have a function test carried out, as opposed to a visual in many cases on the six month service. The six monthly also incorporates a check for changed conditions or layout of the building.
The BS has a list of items (45.3)that should be checked tested every six months and a further page full of tests and inspections that should be tested once per year (45.4) . For example all call points should be inspected every six months but physically tested annually. All detectors should be inspected every six months and subject to a function test annually.