I'll start it off, then.
Contrary to some popular urban myths, there is no requirement for a seperate earthing system (e.g. TT/Earth Rod) at the outbuilding, regardless of the earthing system of the main installation.
The only thing that needs to be taken into consideration is the presence of extraneous conductive parts in the outbuilding and, if required, the CSA of the' Protective Bonding Conductor' that will need to be run along with the supply to the outbuilding.
The option is there to install an earth rod, should this be an easier and more cost effective option.
You then need to seperate your outbuilding earthing system from the main installation earthing system.
The feed to the outbuilding would be earthed at the supply end only.
That should get things rolling