BS5839 part 1 covers fire alarm systems, and allows in some circumstances for some categories of system, for a reduction in capacity of the standby battery if a building is provided with a standby, automatic generator.
Hence if advantage is taken of this relaxation it is appropriate to ensure that the generator is tested on a monthly basis. Further maintenance and testing of the supply from the generator are beyond the scope of BS5839 and, in my opinon well outside the scope of the fire risk assessment.
Checking on Barbour there are dozens of british and european standards relating to generating sets, too many to quote and most will not be relevant anyway.
If there is any doubt about the maintenance then I would simply recommend that the standby batteries for the alarm be upgraded to the full capacity.
