Apologies in advance for the long post, I hope it's not too disjointed.
Right I've spoken to our credit control department here on what the protocol would be, and they pointed me in the direction of a series of guides from the BERR available here:
http://www.creditmanagement.org.uk/berrguides.htm
I've had a quick look, and they are about 4 pages apiece, informative and not particularly wordy (making them probably some of the best documents ever produced by a government department). I encourage you to take a look at number 7 ("Chasing payment") and number 9 (the rather ominous "When all else fails"). In the meantime here is a very brief summing up of how to start things off.
The first thing you need to do is find out the name of the person responsible for making the payments, and send them a letter or email (if you can get their email address).
In the letter you should detail who you have spoken to and what they have said to you with regard to your being paid. Keep it short and unemotional, just a statement of the facts.
Ask them get in touch with you to negotiate, and provide them with a method of getting in touch, a phone number or something.
Explain what action you will be taking next (legal/small claims/contacting any recruitment body they are members with) with regard to chasing payment, sometimes the threat of action is all it will take.
That's all pretty long winded and I would imagine that you are probably more concerned with getting things done as quickly as possible under the circumstances, so speaking from personal experience: If you aren't speaking to someone responsible for making payments (probably the payroll or finance departments) you need to put pressure on so that you are, because they are the people in the know.
They should be able to tell you what the problem is (if there is indeed a problem), and potentially how to fix it. It was very rare to not get an answer from a payroll department, but as a company we had some leverage so that may have been a factor. See what they say and go from there, hopefully you'll have a clearer story.
I hope that was of some help, fingers crossed that the guys here will be able to give you a simpler more immediately effective answer.