Hmm....... thanks again. Deriving a fifth 3-letter word from the 16-letter entries, using the same derivation method as the first four, only seems to throw up one possible candidate, which I can see could be deemed 'thematic'. Unfortunately, it's not in my grid. Ah, unless it's slightly different from the first four (I don't want to be explicit to avoid a spoiler), in which case, I can see a possibility in the grid. Failing that, I must have an error somewhere in my grid.
I think I'll follow Jack's lead and have some dinner before continuing.