Probably need a few more details .. like does the DNO provide a PE terminal - that'll dictate whether the 250m cable is TT or can be TN, and whether the 100A switchfuse will do for ADS (but double check Zs as the armour will be "small" compared with doubled cores, especially for that length) or whether you'll need an RCD at the supply end as well.

Unless the DNO are willing to guarantee a non-PME supply or there's a buried grid you can bond to, the stable end is likely to to have to be TT - but whether the 250m cable is part of the same earthing system will be a design choice. The TT system the TN system (if any) will have to be kept strictly and reliably separated - and nothing exposed and connected to differing earthing systems within reach of each other.

Horses mean section 705 of BS 7671 will apply (Agricultural and horticultural premises) - there's more in there than I can reasonably type here, but there are lots of additional requirements to deal with everything from the brute force and the "conductive liquids" that horses can bring, to the fire hazard from the straw. "Long wheel base" 4-legged animals are also particularly prone to differences in potential across the ground surface - so take care with the positioning/depth of electrodes.

   - Andy.

Thanks for the response AJ it is greatly appreciated, the DNO has supplied a TT system and there is no PE terminal, my thoughts were to earth stake at the kiosk to have this protecting the kiosk and SWA cable. I was thinking of installing a 100mA RCD at the switch fuse to comply with ADS as the fuse will have overload protection and the RCD will have earth fault protection on the SWA cable. At the barn/stable  the SWA gland will be made off to an insulation sheet  and then a separate local earth stake will protect the distribution from this point with all 30mA RCD protected circuits. The Barn/Stable is metal framed and this will be supplementary bonded. 

Thanks for the response AJ it is greatly appreciated, the DNO has supplied a TT system and there is no PE terminal, my thoughts were to earth stake at the kiosk to have this protecting the kiosk and SWA cable. I was thinking of installing a 100mA RCD at the switch fuse to comply with ADS as the fuse will have overload protection and the RCD will have earth fault protection on the SWA cable. At the barn/stable  the SWA gland will be made off to an insulation sheet  and then a separate local earth stake will protect the distribution from this point with all 30mA RCD protected circuits. The Barn/Stable is metal framed and this will be supplementary bonded.