Dylan, once I realised what the long word in Row 6 I did a search in my Chambers CD-ROM for similar items. If you're using the app you'll need WordWeb installed (I don't have it). Then aren't many results. It was a case of matching these by word length and, as Merenz says, to the occurrences of the repeated letter from Row 6.
The main obstacle of course that several of the entries crossing the thematic entries in the first two rows are jumbled. I ended up entering both the coded and jumbled entries one letter at a time - there is a logical route - though others may have found an easier way. That's why I found the final stages a slog, but I think I'm in the minority with this opinion!