Thanks Dr John, I did see it! I may be just being thick this week, but I tried a similar method and just couldn't make it work. Is your four-word treasure C.T.F.C. ? (If you see what I mean!). Also couldn't work out how 10 lines, joining 21 cells (presumably going through some of them without 'turning') could make the right shape! Any further help SO much appreciated!
Think I'm going to have to give up on this one. I spent hours trying to trace the encrypted treasure in the grid last night - but too many possible letter permutations, not knowing the cypher. Ah well.
Sorry QM, this is probably a bit late now but I succeeded by searching for a sequence of letters along the rows with a similar pattern to the last word of the treasure (ie 2nd + 6th letters the same but no others). I hadn't thought of the 'double ring' reference but I suppose with a bit of twisting it might fit...