�850 per week? Where do you work, I want a job!!
Seriously, I'm sure you mean �850 per month.
As has been said, an 100% mortgage will only be granted if the lender thinks that you can afford the repayments. Usually they will lend about 3.5 x salary. So that means (assuming you have no deposit) that you earn between you about 72K.
Also, you will still have to find money for the solicitors fees and stamp duty. And where will you get the money from to put a roof on and make it habitable?