Just to update and conclude this, I went with Aluplast in the end. Just finished the installation.
Early days but I'm very happy. The new units are quite slimline, I was worried about switching from a timber finish to uPVC but against the white walls they're basically invisible.
The top hung window is a massive improvement on the old side hung one. No saggy hinges.
And the simple sliding door is a joy to use. Just effortless.
No signs of any air leakage, even when it's blowing 50mph outside. I'm in a very exposed spot so this was a concern. The old units had started to leak air quite badly. It's harder to shut the front door now because the house is airtight again!
U Values are claimed to be better than the old aluclad units- substantially better in the case of the door, 0.74 vs 0.96.
I paid a bit extra to get the glazing delivered separately, which was money well spent as it meant that installation was much easier.
All in all, very happy with the choice. Not the cheapest, but I've got about as good a thermal spec as I could get anywhere without going to truly silly money and krypton filled units. And it's come in just over half the price of the cheapest aluclad.