Mant thanks ixion. I must have gone wrong, but I can't see where. 24ac doesn't make any sense now if it ends in a "c". I've taken 12dn to be the clue which "suggests the theme", but I still can't make the connections.
Thanks again ixion, and teenieleek. The penny has finally dropped! I didn't know all the names in the first place, which doesn't help. Time for a glass of wine...