Threaten to take them to court for the whole lot. See my answer on the question above.
See this from www.moneysavingexpert.com:
Should you accept a partial offer?
This is a tough decision at any stage in the process. Whilst there�s a temptation to say �fight the good fight and take �em on�, what�s actually more important is protecting your pocket. Let�s say you�ve �2,000 of charges and are offered �1,200; the first thing is try not to think �I�m �800 short,� but instead that you�ve got �1,200 you thought was gone forever.
The ultimate decision is yours. It�s a combination of whether you want the hassle of continuing versus getting the cash. The likelihood is that if you carry on you will get the money, but it will take longer; then again it�s not guaranteed and there�s always a chance (however small) you�ll lose and not get anything.
Sorry to not be more definite, but this is your decision and depends on your personal circumstances, and how much you need the �certainty of the cash�.