One hard-to-spot pair yielded a unique rectangle and it all fell in.
i'll make a start but it's 2 in the morning and I should be in bed
No brain cells were harmed in solving this puzzle. Just as Spellacked said, pair, unique rectangle, a bit of a slog, and voila; nothing beyond an Easy.
I am still struggling. Have come very close to the end several times and found a duplicate. I think I am on total restart 7. I haven't seen said pair but probably the unique rectangle if it involves 5 & 2 or 5 & 1