Firstly you need to establish what kind of load you will be drawing from it. If just casual light stuff ( the odd lawn mower, battery charger etc.) you can spur off the existing house circuit with a 13amp fuse as long as the existing ring is RCD protected.
If however you are drawing high current it would be advisable to in 6mm SWA cable or 10mm, directly from the existing CU to another.
From what you have described I would go with the fused spur running the new sockets as radial
Hope this helps
C.K plumbing services