congratulations on getting your offer accepted!
sorry to hear of your mortgage problems, I wish I could offer some help or advice but I know nothing about that kind of stuff. BUT I'm sure there must be a solution - how did the vendors mortgage it? why would the land count as commercial, there must be plenty of big houses with lots of land (ie a garden) that are classed as residential rather than commercial...
I'm sure someone here will be able to offer some advice, or point you in the direction of someone who can help.
try to stay positive and the best of luck with it all.