Don't think the DNO are going to be much help. As far as I know it's Uk power networks and they will tell you to crack on with a TT system and invariably not upgrade to TNCS. So you're gonna have to figure out these earthing arrangements. Mate there is a big picture of it in the OSG.
As said N fault. Drop the neutrals, meg em out to earth, find faulty circuit, trace it from there. Bell out neutral bars to each other to look for a borrowed N. easy. If both RCDs are tripping you could have dropped a ball and put one side of a ring on each RCD. I don't reckon it will be any...
Don't think the two US marines who died will be "taking it in their stride" the fact that the Taliban were deliberately attacking bastion on the fact that Prince Harry is there.