Hello, No matter what happened,they should have provided you with quotation and agreed a price with you before carried out the extra job and they should have also agreed with you for the price of initial job and if these two conditions applied that's exploitation and it's unlawful. Contract must be agreed before the commencement of work which included price (invoice), so you can challenge this for a fairer invoice. Please inbox me your number if you don't mind. Cheers.