Is it a braked trailer or un-braked?
Looking at what boat you've got I'll assume it's unbraked. A new coupling will cost about £30 from indispention and should just bolt on with two big bolts (which should be there already). Thats for a heavy duty cast one. If you've got one of the cheaper pressed steel Alko ones you can find a cheap replacement for about £14 from Towsure.
Edit: you can get the new style Bradley coupling on here for £22 plus postage: http://www.trailerpartswarehouse.co....prices_27.html
(at the bottom of the page). If yours is an old style this is a direct replcement so the bolt holes will be in the same place as the old one.