Hi Macavity, you are almost there. Sorry if this is obvious, but each letter in the top row has counterpart in the column underneath, read downwards. (Not just one letter)
I was happy with my end-game until I came here to see how others found this fun puzzle - one word and admittedly blindingly obvious. May this old boy have a nudge for the correct solution? I don't submit, having won the pen and pencil many years ago.