This information is credit to M Tippett

Ok, this is a simple job.

Get two standard threaded grease nipple fittings. Get a thread cutting tap
of the same size.

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Locate a spot midway between the two extremities of the trailing arm
(underneath), somewhere in between the inner and outer bearings.

Drill a hole suitable for the tap. The cast iron is thick and it'll take a
while to cut through. Once through the metal, you'll encounter a white
plastic. Drill through that too. It's thin. Don't worry about scratching
the crossmember inside, it's not important in that area.

Once the hole is in, clean it out (burrs etc) and then cut the threads.
Then screw each grease fitting in (one per trailing arm).

Pump each full of grease. It'll take at least one full large cartridge to
fill up the space between the two bearings, each side. It's not designed to
hold grease so it takes a lot of volume! Pump until some grease comes out
of the inner seal or you encounter resistance when pumping.

It should be topped up every year or so. The main purpose is to keep grease
in the bearings (they dry out) and also to exclude water and salt.

Cheers
Mike