So. How can I resolve this matter quickly and with the least expense?

The best way is to try and reach an agreement that is fair and reasonable for both parties and in the best interests of the children of the relationship (if there are any). Try to avoid conflict. Try take a longer view of the matter to avoid petty arguments with your former partner. Be honest and upfront with your solicitor regarding financial and relevant personal matters so as to avoid back and forth correspondences disputing facts