I'm a plasterer who's just gained a 175 visa, I didn't have any formal qualifications, I had to do an AQF 3 (£2200) then get my skills approved (tra) it was a pain in the arse but worth it in the end. You need to have evidence like tax bills, work related receipts, letters from references and customers, photos of work, even photos of my kit. In total it took 2 years and now about 6k, but now we have the visa so money well spent. Try John at immigration2oz he sorted me out.