A company we are in legal disputes with has changed its name. The company numbers are different. All assets have been transferred to the new company and the old company now has no assets. How do we stand?
A company is a legal person, and each company is uniquely identified by its registration number. Accordingly, you are dealing with two quite different companies here. This is not, in legal terms, a change of name.
One of the principal purposes of a limited liability company is to limit liaiblity to that company. It is sometimes possible to "pierce the veil" of incorporation, so that liabilities are attached to some other person (typically a director), but that is the exception not the rule. For general background info, see:
Whether you have any recourse in your particular situation will depend upon all the facts and evidence, and you would need to consult a solicitor, who will need to review all the documents and the matter history, to get a reasoned view upon that question.