Useful links:

The EU Single Market:
http://ec.europa.eu/internal_market/qualifications/index_en.htm

The Professional Qualification Directive:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2013:354:0132:0170:en:PDF

The Services Directive:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32006L0123:EN:HTML