Another update. It's all working now:
https://xpander.mattmillman.com/file...ll_working.jpg
* NICAM is working
* Pattern generator sorted. Now have colour
The hideous rendition of the colours sent me on a bit of a wild goose chase. It turns out that's actually a fault inside of the Arcam Delta receiver. For some reason it is losing most of the chrominance signal. A second receiver I tried doesn't have the problem.
On a side note - I noticed there was a discussion about the legality of "TV senders" well actually I had one of those (purchased from a UK retailer) and I've attached a couple of spectrum analyser shots of its output versus this setup.
The TV sender transmitting a DSB for the video carrier, and also mirror image of the audio carrier occupying two UHF channels. That looks like a good enough reason to ban such devices to me.
The setup I show here outputs a beautiful 8 MHz VSB with no artefacts from 0 Hz to to 3 GHz. Very happy with that! Anyways. That's enough on this topic.