FTB here so quite nervous about the whole process and looking for some advice form the more experienced.
We have an offer in place, a mortgage agreed and are now just waiting for the bank to sort out the valuation, the outcome of which I believe dictates the final mortgage offer.
How likely is the valuation to be lower than the asking price?
The AP was £85000, our accepted offer is £83500 but if the valuation is much lower the mortgage offer will be reduced and potentially we're screwed 
Any words of warning or comfort as I face 2 weeks of nail biting?