If you're going to allow anyone from that firm to set foot in your house again, then I suggest you at least get yourself one of these, so you can show them how appalling the work is, without any room for them to argue the toss about it. Holding a feather edge up to the wall, right in front of them should really embarrass them into admitting it's substandard work (unless they are hardened cowboys, accustomed to pissing in customers' pockets and insisting it's just raining).
www.toolstation.com/draper-feather-edge/p40959
www.screwfix.com/p/forge-steel-feather-edge-72/52912
(any decent builders merchant will also sell them)
Good luck, and
stand your ground! That work is
definitely unacceptable, and don't let anyone try to convince you otherwise.