And it broke again
Very bizarre really: I left it overnight installing DR3 on disc, and downloading all my other games. I went to try it this morning and it wouldn't recognise any disc at all, nor play any game that didn't need a disc. I gave it 10 mins of faffing, then went to work.
Spoke to Xbox support (the same guy, he rang me actually to make sure everything went okay, top service) and he said that it sounded like I'd got another duff one with disc drive issues, which isn't unheard of. He's going to call me back later to run through a few diagnostics before sending me out another unit.
However...
I came home and thought I'd give it another go, but this time shut it down properly not using the quick start feature, and lo and behold it's all fixed! Everything works fine, discs are read as normal as I'm installing Ryse as we speak, and I can play games again. FYI, when you load a game there's a small bit of time between 20 secs to 1 min whilst it downloads your saves from the cloud, then it's as if you never left it.
So in short, if you have an issue then call Xbox Support on 0800 587 1102 and they will bend over backwards to help you out, don't bother with the lying turds online
Now to plug my wheel in and see what Forza 5 is meant to be like