Oh this one was very frustrating. I could see what I had to do, and I even had the correct way of resolving ambiguities. But could I get the geochecker to go green? No.
I met the CO at one of the Surrey Meets and he confirmed that my approach was totally correct. I emailed him my workings to be marked - I realised it could only be a arithmetical error on my part.
Within a few minutes I had a reply. Was it my maths - no! It turned out I'd made a "spelling" error: I was using Chobham instead of Cobham. I've always confused those two. So suitably chastened I recalculated the numbers and this time it worked.
An easy find at the GZ - we're getting on a roll now. Took nothing (the cache was empty) but left a bottle of gemstones.