Ok, fourth time lucky!
Just to recap: ordered in June on first day, it quickly developed terrible IR. After a lot of correspondance with AppleCare online and Apple ER I eventually got an advance replacement arranged. It arrived but the screen was as bad as the first, dimmer and more discoloured / unevenly coloured. Sent the second back immediatley, held onto the first for long enough to do a business trip (appreciated them letting me do this, even if it took 100s of hours to sort this out.) They only offered a refund at this point, so I took it.
Waited a few months until December. Still wanted a machine of this spec, and felt that there were less reports of problems so it may be a good time to try again.
The machine was another LG, with medium IR which I decided to just live with. But after 2 weeks it developed a vertical line of dead pixels.
Finally got another appointment at the local genuis bar (the same guy who I'd taken the first laptop to in July, luckily, so he was sympathetic.) They offered to replace the screen, and even though I'd refused last time (due to not wanting a brand new machine torn open), I decided it was my best shot this time, being just out of the return period.
To my amazement, they replaced it that day and today it has a new Samsung screen. It's brighter (I think, or maybe the whites are), a touch warmer in colour (yellower perhaps, but closer to my Dell Ultrasharp in colour, which has seved me well.) The screen is nice and even. Text looks slightly sharper to my eye. Blacks are noticably muddier / greyer, colours are a touch more desturated, which is the one thing I do miss (always found Apple laptops a bit washed out and the Retina LG was definitely better than any I'd seen before for that.) But all in all it's a better screen just because there's no IR, which was very frustrating when trying to do detailed edits to artwork.
So, the reason I'm posting all this is to suggest that it's probably worth taking a screen replacement, even if it's really new, over trying the machine replacement / refund lottery. I think the rumour that most Samsungs are ending up as backups for replacements is probably true. Wish I'd done it back in July, and then I'd have had a working computer for an extra half a year!