Dave, I understand 4.5.1 is relevant here and that will indeed help the OP in simplifying his inspection and testing, but I am struggling to understand how the OP's scenario could be outside the remit of BS7909 altogether, please can you explain this?
Digressing slightly, 6.2.1 could also come into play - a probable situation if the OP is going to attend private events where another generator will be powering the marquee, toilets & entertainment.
If a caterer advises they don't need any power, most power providers will assume they are bringing gas. When said caterer then rocks up on the day with their own genny, and plonks it close to the set already up & running, this will likely impact on the existing 7909 details of supply signed off, as the METs of both sets may need to be connected.