Yes, 500 quid cash, and they'll be pleased to have got away with it, CW.
Not exactly a difficult job. One where it's pretty easy to foresee the problems, and allow for them in the quote. These people have behaved like amateurs. The original estimate is a nonsense. Even from here, I can see the possible work involved to do a decent job.
Under-quoting is always a sure sign of amateurism. They're simply afraid to offer the real price, thinking they won't get the job because of it. Then all the nonsense starts.
Possibly perfectly decent guys... not bandits. Just totally un-businesslike. These people just invite hassle. Don't have them back for the bigger job.