Hello my name is Alex and I will help you with this. Please note that I am a working Solicitor and may be on and offline as I have to attend Court and meet with clients, even at weekends. As such you may not get an instant response when you reply, but rest assured I will be giving your question my immediate attention upon return.
You should write a letter recorded delivery asking for payment for the sums that have been agreed and are not disputed and ask for it within 14 days or say you will go to Court. If they refuse then you can issue proceedings in the County Court. You are entitled to be paid a reasonable sum for reasonable works done. If they do not pay you then you can issue proceedings either by completing form N1 and submitting it to your local County Court http://hmctscourtfinder.justice.gov.uk/courtfinder/forms/n001-eng.pdf or by issuing an online claim at: www.moneyclaim.gov.uk
However please remember to keep a copy of the original letter and send it recorded delivery. The Court will then issue a claim and if not defended, you can enter Judgment and seek enforcement or if defended the Court will list the matter for timetabling for trial.
If this fully resolves your question then please take a moment before you leave and rate my service, it only takes a second and is an important part of the process. Otherwise if you need more information or help then please click reply. Alex