Thanks for the suggestions, guys.
After careful consideration

I realised (a) there's no point being too precise about the thing unless I take
very careful and difficult measurements of the exact degree of bend on the offending pin, and exactly which bit of it has a centre which lines up with the other pin above it, and (b) they aren't my damn gates anyway, and I won't get paid more than a couple of

.
So I've made a sleeve with plenty of clearance and a tapered bore to fill up the gap between the oversize hinge and the pin, so the two of them can make their own minds up as to how they fit together.
To make this and another five sleeves (my pal has double gates at the front, and a small one on a side boundary), I used a part painted, part rusted piece of what looks an upright from some school railings. Dreadful stuff to turn/drill/bore.
Andy