I have PhD interview next week, but I'm not sure what to wear.
Would black jeans with a blouse and jumper combo be smart enough?

Whatever you're comfortable in. That would be fine in my department (psychology).

Yep, don't over do it as it will be out of place - they will be smart/casual, and more on the casual side.

What image does your PhD supervisor expect you to present at conference?

If that's something you don't know then I would play safe.

As a man, I opted for shirt and tie (as much as I hate ties) and feel I made the right play if only to get the PhD position over the line. After that, I was able to work out what was the right clothing for each situation.

Mostly, no one cares and all is very casual but conferences and presentations can be more formal.

I suggest trouser suit unless you know what your potential supervisor expects, then once you have your foot in the door you case assess potentially formal situation on a case by case basis.

Ian

Have to agree with others, wear what you're comfortable with. I tend to wear dress trousers and blouse for interviews/conferences and I'm ALWAYS overdressed but it's nice to show you take it serious. Good luck!