My deposit is 15% and my mortgage will be massive. My bank don't have an issue with the size of the mortgage, as they approved me for a similar one last year on my purchase which fell through.
One of the challenges they have us that Land Registry have got a backlog on public ally showing properties on the register, so comparative properties which sold for more or similar aren't on there yet.
I don't have any more money to throw at this purchase if I get another downvaluation, however it is a small 2 property chain, and vendors have a similar timescale to complete (school summer holidays), so I'm hoping they will agree to drop to the difference, whatever that turns out to be.