HI everyone
Just joined the forum after my first read through of all the clues.  I got far more answers from my first read through than normal, but no "pairs".  Thank you Mikey and Chris for giving a broad hint for 26ac.  I'd googled sea snail and got nowhere.  I'd never heard of it.
I'm off for second read-through now and will be back for some hints later.  I'm relieved to hear that the "pairs" are of equal length.
Is 18ac an anagram?  If so I can't find it.