Ingenious construction. I spotted the derivative in the grid long before collecting all six letters, and thought of the "non-perimeter cells", and thought, no, surely, he hasn't, he couldn't..., could he? He only went and did! Like others I found a few of the clues a shade unnecessarily stretchy, but many of them are fine and some excellent. I found the NW corner tricky, and like will37 found the thematic grid helped me slot the final letters into place!
I'm still struggling to parse two. The definitions are clear, and thanks to the grid I am certain of the answers! But the wordplay is eluding me for 1d, Duck again rising more steeply, soaring clear of a rook, and 4, Scots market Ayrshire course: get first round free. Also, I know what the prototype is by working backwards, but haven't found the fifth letter -- and I don't think it can be from one of these two clues! Can anyone nudge me to full closure?
espebe, I too have requested and not yet received. When the notice appeared in past years, John was all ready to go, but bear in mind there's a new hand on the tiller so a little feet-finding time may be in order. The email address is new, and if requests can be made by email an auto-response so we know our email has actually arrived safely would probably be in order, and may be something we can suggest in due course.