fb,
Hmm. Been there and done that! Really annoying and not necessarily easy to deal with, in my experience. Trying to be sensible about it is more difficult, if like me it makes you simply mad. It is never wise to let too much emotion into the decision-making process.
I guess it depends upon your situation. Do you have plenty of work from others, or are you scratching around to pay the rent? Are there other outfits out there who would give you work, if you make yourself available to them? If so, make enquiries and then just move across, having carefully agreed terms with the new outfit before you go.
Why are they paying late? Is it because they are on max overdraft from the bank? In that case they are keeping your money due for last month to pay this month's wages for their employees. If that is where they are at, find another outfit to work for. They may not be there at all in a few months time.
Someitmes some smart arse in accounts just thinks it would be a giggle to pay late - he thinks he will get extra browny points for being able to reduce the company overdraft at the bank. If it is more likely to be like this, then it is worth approaching your normal contact and asking what is going on. Normal people generally appreciate your position, although there are some real s***tes around in this game from time to time.
It will help if you have something written down and agreed between you as to payment terms, as well as the other main elements of your contract.
Good Luck.