In your typical month ...
Your Money in is about £13000+VAT - the VAT is about £2500
Your money out is £10813
Therefore you have a viable business if your clients pay their VAT like they're supposed to.
Your problem, as others have pointed out, is that you are letting people underpay you. If nobody pays their VAT, your money in becomes not £13000 PLUS VAT but £13000 MINUS VAT, as you have to pay their VAT. Then your money in is only £10500 and you're going under.
Two solutions to your problem, as long as you want to stay in business.
One is proper credit control. If you can't do this yourself, consider outsourcing it.
The other solution is to charge people less. If you charged people £11000+VAT AND THEY PAID THE VAT, their total bill would be about £13000. If you charged them £13000+VAT and they DIDN'T pay the VAT, they would STILL be paying you about £13000. The difference between the two situations is that in the first one, you can afford to pay your VAT bill - in the second one, you can't.
That said, I would strongly advise better credit control as the solution to your problems. We do our own credit control but, if you're looking for a third party provider, I have heard good things about
http://www.directroute.co.uk/