Dan
I had same problem a few years back with a company. Took the Citizens advice route. I had a contract of employment, so on the advice of CAB I compiled a letter stating he had 'X' amount of days to respond to why I hadn't got paid on time, after which he would be breaching my contract of agreement & then I would take legal advice to move it along...
Turned out he really didn't give a stuff! Was a Complete T***ER! & I Duly walked out of the company without giving any notice of leaving once that Month's money had eventually gone into my account...they too were a Big Company with Big accounts & went bump 3 Months later...
Hope you get it sorted...