I've been 'carrying on' since Sunday but the final step is still beyond me. I have solved all the clues and have the 4 thematic entries. I am assuming that a letter is removed from each of the 5 answers which are too long for their cells and added to the thematic down entry to create an element found in that. I can make that work for the eighth row but nowhere else. As usual I am probably over-complicating things.