Say for example if I withdrew £50 from a cash machine with credit card, any ideas on how much interest is paid at the end of the month!?

Most cards are about 24%

You pay at your card's "cash advance rate" not the "purchase rate". There is no grace period, like with purchases. Every day a cash advance is outstanding accrues interest on the amount outstanding on that date.

That depends entirely on the card and the bank that backs it. It should be in the literature you got when you got the card.