There is a code of practice issued by BSI and endorsed by HSE and other info from HSE on construction site distribution and management of generation sets.
The HSE stuff is on their web site foc, the BS you will have to buy.
Just like 7671 if you are doing this work then you must have a copy of this standard, just like the EM lighting & fire alarm sandards if you are doing that work.
I can't remember the number of the BS but it is entitled something like "code of practice for elecrtricity distribution on construction sites" 7375 rings a bell.
Please remember as it is designated a CoP it will have greater legal standing then 7671.