Perhaps I'd have done better not to 'correct' my partner's 3♥ - with hindsight.
Against 5♣x, obviously a spade lead is best here, but understandably N led a ♦. Declarer played Q from dummy and ruffed South's K, then laid down A♣, discovering the 3-0 break and the missed opportunity to finesse.
I think it was at this point that declarer apologised profusely to me, and I responded with something like "all right but let's wait till the end of the hand". I think I shouldn't have said that - maybe it put partner off their play! I should have answered something like "no problem partner". Poor etiquette can be costly, as you'll see.
Anyway, declarer played a second trump, which South ducked, then a third trump to the K. I might not have done this, I tend to leave a single outstanding master trump at large, but it was probably the best move here.
South helpfully switched to a ♥, and now I saw that this rather dodgy contract could be made! So long as the hearts aren't 5-1, declarer puts up the 10. If North covers, win the Q, then throw two spades and a diamond on the established hearts. If North ducks (or if S has the J), win with the 10, ♥ to Q, ruff a ♦ and play off ♥ as before. Declarer loses just one trump and one ♠.
Alas! this wasn't what happened. Declarer played the 7 on the first ♥, North covered (naturally) with the 8 and all was over.
I feel that, even though as dummy and powerless, I was partly to blame. If I'd kept quiet declarer might have found the winning play.