Hi i work in retail, not in m&s but very similar, and if that were a pair of our slippers we would change them straight away and it wouldnt be a problem, i'd give your local store a ring first and explain that, you just want to change them, explain about your hip and they will probably pay for your travel expenses, thats what we would normally do anyway. i hope you get them all sorted and theres no need to kick up a fuss,that just makes you and the staff uncomfortable and the whole situation worse.
Jenni