I applied for a PhD and was actually invited for a 2-day interview - is this normal? I read through this board and some people were offered a place without an interview at all, other had 40 minutes interviews .... I wonder what on earth they will be doing to me in 2 full days?

they check you can stay awake all night. A highly valued skill for you phd

Probably you'll have a whole day of meeting supervisors, viewing facilities eyc. Make sure you have some intelligent questions on hand to ask them on that day - they will be assessing you even if it does seem informal!!