So, whipped the links and bushes off, with a bit of angle grinder action thrown in of course to get the old links off.
It was clear when I popped the links off the balls that the problem had been water ingress into the ball/socket joins on both ends of both links.
That got me thinking - how do I prevent the water getting in here?
My experimental answer is to use some cut-up sections of bike inner tube (big mountain bike stuff), filled with grease, and tywrapped on. I could have used one long section to cover top and bottom joints in one go, but that would have been awkward to assemble (and I didn't think of in until I had finished!)
Now, I may have designed a perfect water keeper outerer, or I may have invented a water keeper innerer. Only time will tell. Each bit of inner tube is well filled with grease, and the bits of tubing are more to keep the grease in place than to actually keep water out - the grease will do that itself.
Anyone else tred this? Success or failure?