Some progress I think. I'm pretty sure I now have a leaking waterpump!
I think the radiator is fine. When I had it running the coolant hoses weren't being pressurised. I think water is leaking from the pump and on to the fan somehow and then dripping in to the bottom of the fan shroud and leaking out from there.
This is it after drying off the area and pressurising the system with my DIY cap:
I'm now happy that all the work I did previously was done OK - there's no evidence of coolant from anywhere above the water pump (valley gasket, throttle body heater bypass pipe etc). There's also no route that I can see that would let coolant reach this spot from the head gaskets.
This is a video of it seeping very slowly with no pressure in the system:
This is what the engine bay looks like with it running in this state:
It's hard to tell from the video but it's billowing out from under the rocker cover.
I'm now wondering if the "steam" coming from the exhaust area is actually SMOKE. Specifically, smoke from this:
I've clearly gotten something wrong at the back of the engine bay there. That hard plastic tube has slipped to rest on the heatshield, melted through and is now two separate pieces of pipe. Each of them is now glued to the heatshield :(
However, I'm not sure how the smoke from this (if it is smoking) would get from there to billowing out towards the front of the engine.
Anyone able to tell me what they are? My first thought was gearbox/transfer case breather pipes. They look similar to the breather pipes for the fog lights at the front of the engine bay. I'm not sure though.