If the debtor is truly insolvent and the case is a cant pay rather than wont pay situation then you may have no alternative but to allow the company to enter liquidation. Accepting the £30,000 may be your best option, though I would not advise to allow this to be in full and final settlement, you should accept the payment as a deposit for the rest.
If the company is not insolvent and simply wont pay, then you should take them to the civil courts. A county court claim would be cheapest, but they do not have the legal authority to enforce judgments of over £5,000. If the company is not insolvent, then any IP they approach is under an obligation not to allow the company to be liquidated. Though the definition of this legal point is quite wide. You could try a solicitors letter of claim, which is the beginning point of any claim to ascertain what their response is when actually threatened by
If the company is insolvent, then it would be a false economy to bring a claim through the civil courts, as you would come to costs that would not get you anywhere.
A good way to ascertain as to whether the company is insolvent or not is to check their companies house accounts, which are public information. Also obtaining a credit reference for them is a very good indication. As i mentioned previously if you PM me the company name i can investigate the company finances to help you come to an educated guess as to the companys potential technical insolvency.
I would be wary in this situation, if the company is truly insolvent, how does it have access to £30,000 immediately?