As the title says...

They seemed to ask me very easy questions, all of which I *think* I answered ok. But I was only in there for 20 mins (with 3 interviewers) and they didn't ask me anything challenging, which makes me think they'd already decided I wasn't a good candidate..

Ugh.

Don't think you should necessarily abandon hope. If it was an advertised post (rather than a meeting with just you in response to a speculative letter) the Uni HR will have set in stone that every candidate must be asked the same questions to avoid accusations of unfairness and they wouldn't have bothered interviewing you if they didn't think that you were a plausible candidate, probably they will have had many applications.

Thank you for your kind words! The thing that concerns me is every time I answered a question they would nod and agree.. In all my previous academic interviews/poster presentations the interviewers picked on little errors, made me defend my work and generally got me to a point when I had to admit I didn't know the answer.

It makes me think they'd already found a perfect candidate and were just going through the motions. Which is really disappointing as they'd seemed like genuinely nice people who'd be good to work for and the project was exactly what I wanted to go into.

At the end the lead supervisor said it might take them til next week decide so not to worry if I didn't hear for a while.

And yes it was an advertised post, up for a long time and available to any health/ biosciences student!

I don't think you should be discouraged by the fact they did not pick up on little errors. From my experience it is unusual for this to happen in academic interviews. As an interviewer, I might have asked for clarification of points in a presentation, but I would not have asked you a question and emphasised the errors in your answer. I suspect the interviewers will have made notes on each candidates performance and allocated a score (explicit process insisted upon by HR). On this basis the best candidate will be offered the post. The reason for the nodding etc is to encourage you and keep you going, as the interviewers are aware that interviews can be stressful. By nodding I find candidates feel at ease and will expand a lot more on the point they are making thus showing the depth and breadth of their understanding.

I think 20 minutes is a normal length for an interview so I wouldn't worry about that! Often I've found that interviews have a set of 'key phrases' they want the interviewee to say with each question, perhaps when you said the right things they were all nodding in agreement that you had said the thing on their tick list. It sounds like you had some tough previous interviews and from my experience it's not the norm to try and trip you up so don't worry too much about that not happening in this interview.

Maybe they are running for other things like meetings ect.? these professors can be very busy... also, maybe they didn't prepare a lot of questions for you because they are busy :P
Don't worry too much, best of luck!