Hi @pastaislife. I couldn't bear to read and run as my partner & I were in exactly your position just a week ago & it is truly horrible. Especially when you have your heart set on your new home!
Like you, we had our mortgage application in with a high street bank and they were due to do their valuation of the property. It came in at 10k under what we had offered (and had accepted) on the property. Luckily we have a 30+% deposit so the devaluation didn't actually impact our mortgage offer.
We were shocked by the devaluation and didn't really know what to make of it. We knew the house we were buying was priced a little bit more than similar houses in the area, but it had more going for it such as being a corner plot, anextension outside, newly fitted ensuite & kitchen etc.
I did some digging and like you, the bank was only doing "desktop valuations" which basically meant that someone would sit on Zoopla, which is notoriously inaccurate, and decide what they thought the property was worth based off nearby sale prices which, in our case, were 2+ years old! The reason that your bank cannot offer your a proper valuation report is because nobody actually visited and inspected the property in person to really judge it's value.
If it's possible for you to proceed with the sale regardless, I would urge you do so. My partner & I just received our survey back this morning, which included another valuation, and their valuation was far more in line with what we have paid so we feel very relieved.
If it's not possible for you to continue with the sale because of deposit funds and you really love the house, then you should contact the estate agent and explain the situation. I am sure they will be able to speak directly with your bank regarding the valuation and go from there in terms of seeing if the seller can renegotiate the price. Please have hope! It's highly unlikely that the seller will refuse to lower their price given that whomever comes in after you & has their valuation will likely have the same outcome. Banks are being notoriously cautious with their lending right now because of Covid 19 as they are pre-empting a financial/housing market crash.
Fact is - if renegotiating with the seller is your only option, the worst they can say is "no" so it's worth a try.
Good luck OP! Keep us posted please!