Some on-topic closure:
Went out to the garage this morning. Pulled the bleed kit from the box and had a go.
Here’s what I did...
Remove reservoir cap screw and screwed in the funnel thing. Removed the stopper from the funnel thing.
Remove dust cover from bleed nipple.
3/4 fill syringe with fresh oil
Jam hose end onto bleed nipple
Discover the bleed screw had been tightened by an ape.
Get socket set and 3mm hex. Get the bleed screw loose.
Push fluid up the system until the collecting funnel was half full.
Close bleed screw and pump the lever. Decent feel and pressure. No leaks at caliper.
Cycle lever a lot then hold in and loosen bleed screw pushing out extra air. Ala bleeding automotive brakes. Repeat until no air.
Close everything up.
Test ride.
Win. Rear brake works. No leaks from the fitting (after it had been tightened).
Brakes are still honking a bit but it might come good. Either way BrakeKleen and new pads next time I’m ordering something will solve it long term.
Expect a post in the Fuckwit thread when the fitting starts leaking again and I just buy new 4-piston calipers instead