This is the bit I couldn't find.....
45.3
"...........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."
So yes you can give a certificate - providing you have done a 100% "annual" test on a system that may not have been checked for years, but back to the OP, if you have a contract to carry out maintenance to 5839 and the customer doesn't want a six monthly test then surely you need to qualify the certificate saying "it doesn't comply with BS but it's only a recommendation" etc etc. - Your RA mr client, do as you wish and make sure you tell your insurer....