Whew! Got up this morning, rearranged a couple of letters and finally made sense of the wretched thing!! Never have I done such a hard puzzle. Now I can look forward to enjoying Christmas! And a Happy Christmas to you all!
PeterB - you're looking for a 2-letter abbreviation for special, plus a word meaning "corroding process". For the latter, think copper. The pre-encrypted word is obscure, but in Chambers.
Many thanks peterm - I thought I'd tried all combinations of the potential decryptions for the letters that I had - rather obvious now that I hadn't tried the one combination that gives the answer. I can now have a shower and get dressed!