And for anyone who wonders why they go, it's not just through age with the rubber perishing, it's because the sleeve

(number 4 on there and which hasn't come with the replacement drop links that I have and I suspect doesn't with others???) corrodes and bursts the bush from the inside;

And without the sleeve (or with it worn/corroded) there's a lot more lateral movement transferred onto the bushes, when they're only meant to take vertical changes in load. You could easily over-tighten without the sleeve, too.
What's needed is a stainless sleeve.
I'll see if I can find some bar stock the right diameter that I can cut down, otherwise I'll be asking if anyone on here can turn some for us

Approx 13mm OD (I'll measure the ID of the bushes and they can be matched)

Approx 8.4 ID (probably originally slighter tighter than that, but it's an 8mm clearance hole)

Approx 26mm long
For now, my old sleeves will have to go back in as a temporary measure

(with lots of grease to stop the inside of the bushes being eaten!)