Window company cant do agreed work now want more cash

I was quoted £17000 for a replacment conservatory, the new one would have a dwarf wall and super glass? the survayor cam and said that the dwarf wall cant be built and i would have to have all new foundations. i would assume this was in the price as we never had dwarf walls to bein with. i was quoted £6000, i spoke with other companies and researched and should get it for £1000. i beleive they havent honored the agreed contract but im scared i will lose my hefty deposit of £4500