Ah, thank you so much everyone! 
There are some amazing pyjamas on this thread. I'm getting pj envy and am trying to resist temptation to treat myself to some new pyjamas now...
@Sickoffamilydrama I usually wear a 5 and have these in a 5, they fit, but there's plenty of room so if you're on the cusp of a 7.5 you might get away with the 7 if they have that in stock? 
If not then Raid London have similar, but not in lilac that I saw, and Zara have some clumpy lace up boots in a lilac colour in their sale (I shopped around a bit before committing to these!)
@AnnieSnap that's so kind of you, thank you! As @EllaPaella says, the Aimilitas are much softer leather and don't need any breaking in at all. Mine are the comfiest boots I own and I've walked miles in them from day 1. With the originals, I tend to buy second hand in the hope someone else has done the breaking in for me, and they're super comfy once broken in. I know Schuh stock DMs, not sure if they have the Aimilitas, but might be worth popping in if you have a store near you and wanted to try them.
@OublietteBravo thank you! They are part of the H&M Group, I think, yes. The sizing is very random! They come up big, IME, a small is a 10-12, for example, whereas I wear medium everywhere else but a medium is big on me there, and I need an XS in some of their oversized things. I'm a 10-12, more of a 12, but tend to buy a 10 in Cos. I do think their clothes fit beautifully when you find the right size, though. I've had very little from there that I've sent back, even when buying online.
@EllaPaella thank you! I do love them so I'm going to wear them on the nursery run (about the most exciting place I go every day...) with pride!