At the weekend I was ready to remove the final old socket and insert my loop of extra sockets, so I'd isolated the CU, did a check L->L, N->N and E->E around my new loop which was OK, then I did a similar check around the existing ring at the point I would join into it ... live and neutral OK, but oh dear, no continuity round the earth!
Spent bloody hours over the weekend (in bursts of a few hours so the fridge/freezer wasn't off for too long at a time) with temporary shorts between E/L or E/N at various points to trace where the circuit went, and whether the fault was up or downstream of those points, checking junction boxes under under floorboards which I knew were there, finding one I didn't know existed, remembering sockets that were behind wardrobes that haven't been used for years etc, what a ball-ache.
Finally today traced it to the cable on the ring that's between one socket in the back bedroom and another socket in the front bedroom on the same wall, cable goes down wall, under floor, back up wall, it's plastered over, can't see if there's metal capping over it, but I imagine so, similar cables in other original parts of the house are, I've never done any work along the route of the cable, and can't see where any previous owners have either.
There's a convenient loose floorboard (I presume from when the central heating was added) about midway between the sockets, where I can poke a torch and a mirror on a stick down, no damage to the cable where it's visible, so I conclude it has probably been like it since the house was built in ~1976.
Getting access to the cable at either end is a bit of a problem at the moment, back bedroom is chock-full of stuff while I'm working on many other parts of the house at once, front bedroom there's a large wardrobe in front of the socket which I can barely move, and there's nowhere to move it to at the moment.
So for /now/ I have put it all back as it was, knowing that every socket is earthed, some from one direction, some from the other. There's nothing which is a "high powered" appliance on this ring (I put a second ring in just for the kitchen and garage a few years ago) in fact it's probably only the fridge and boiler which aren't double insulated appliances anyway.
I suppose I could deliberately split the ring at one of the junctions, and replace the 32A MCB with a 20A MCB and call it a radial instead of a ring (that just happens to have the CU at its centre).
Not having a trained monkey to assist with going round with the multimeter while adding/removing shorts made it a slow job, in fact I thought it might be worthwhile knocking up a battery powered box to toggle a set of relays in a sequence, so you could wire it in and it would give e.g. one brief short from L/E followed by two longer shorts from L/N to cut down the number of trips up and down the stairs and squeezing behind furniture etc, does a similar testing device exist?