Ahh I see - I think the bit I was getting lost is that we would still be able to get the necessary mortgage with remaining deposit. It would just be a worse interest rate - we wouldn't necessarily be completely out of the purchase.
I added a table - it does look challenged if the valuation fell below £340k, as you would probably struggle to find a lender who would offer less than 10% LTV
Any shortfall on the value you would need to make up with cash.
Worked example:
You could probably find a mortgage at this LTV, but you would obviously have a higher interest rate.
I have done you a sensitivity table: