Alexander McQueen oversized sneakers fit

So I’m a size 9/10 UK depending on brand. Purchased these sneakers in a size 9 and they fit well in most places however my heel slipped quite a bit whilst walking in them, so I returned them for a size 8.5. The 8.5 is a lot tighter, toes are touching the end of the shoe but even with these ones the heel slips a little.

I love the sneakers so it’s really frustrating but I don’t know if this is just a case of the 8.5 being the right size and they’ll stretch eventually with wear.

Any advice? I only have a week left to return these for a refund.

Cheers