Hounddog's suggestion is a good one, but could possibly get you into further trouble...
It sounds like they are taking the p*ss and make a habit of this sort of thing. Personally, I'd take them to court as I think you have a fair case and the price you charged for the work doesn't seem unreasonable. Sounds as if you didn't give them a quote for the extra work, and in any case, an estimate is only an estimate.
In the past we have taken a few cases through the small claims court, which was a fairly simple procedure, and we did get the money eventually. Good luck, hope you get your mon