They were really good - Dell's support & service are genuinely great. The first person I spoke to ran through the usual flow of 'check this / check that / change this setting' but was basically trying to work out if the monitor had a fault.
I was then put through to someone who essentially said that I'm not the first person to call in and report it, they have had some success with others by replacing the monitor for a like-for-like switch but they also are aware that for some users, the panel type just doesn't mesh with them and they'll offer a refund. She also asked if I was suffering with any flicker but I said, for me, it was purely the eye strain and that I'd not really noticed flicker.
I said I'm happy to try a replacement to see how I get on so that's coming out this week - but she said it won't be an issue to arrange a refund right away either if it doesn't make a difference. Really good service from Dell, in all fairness.
This is on SDR with brightness at 35%!