In my previous life I was also self employed, I wouldn't take work from people who got more than 30 days overdue. I would inform them that I considered any bookings while money was outstanding to be unconfirmed and therefore I would take other work. If they wanted a reliable person then they had to be able to be relied upon.
Something that worked with one difficult client was to send them a letter informing them that since I had paid the VAT on their unpaid invoice to customs I needed their VAT number to inform HM Customs who the bad debt was when reclaiming the paid VAT, I got a cheque two days later. Only works if you're VAT registered of course, and they're not wanting anyone official to look into things.
If it gets past your terms, and there is no sign of the money being forthcoming then sign up at moneyclaim.gov and get the courts to sort it out.
Good luck.