I've now cleared the blockage. It was in the lower tee (circled in my first picture). I decided against dropping out the pump, as I was a little unsure about fitting it back again without leaks. So I cut the pipe below the top tee and had a nosey with a screwdriver. That was all clear, so I cut above the bottom tee (removing the whole section of pipe) and poked downwards with my screwdriver. It was completely blocked and very tough, but I smashed it with my screwdriver.
After patching up the pipe, the system refilled fine and heating/water are back in action. I put Sentinel X400 into the system and plan to drain again in a few days, after which I'll put X100 in.
However... now I'm suffering from a few noises in the system. I get some humming that comes and goes. Some gurgling noises around the pump too. Any suggestions? I tried turning down the pump speed from 3 to 2, which has possibly helped a little. Prior to that, I've bled the system from all rads (several times) and bled the pump and the valve that's next to my cylinder (top left in my first photo).
I'm imagining big lumps of crud now circulating the system, killing my pump... Other similar threads suggest replacing the pump - do I need to do that?
If you can afford £100 or so, this is a good time to add a magnetic filter. There will be disturbed fragments of the hardened sediment, and lots of loosened sludge, circulating.
Some of the noise may be air, which will probably settle out after a few days of normal on/off use. If your pump gets very hot (hotter than the pipes it is attached to) and stops circulating, then it is jammed or dry. You can have a go at dismantling and cleaning it, but (I am a DIYer not a pro) when I tried that, the bearings were already damaged and clogged with cooked-in sediment.