Thanks cloverjo ! I spotted the HYPO/HYPER potential in the title before realising I must have incorrectly attributed the change in 19 down to one of the extra words. Turned out to be 18 down, which I still don't fully understand.
The 8 clues I have with extra words are:
22, 24, 26, 36, 38, 40, 23 and 25
Which is the ninth please ?