Always try to resolve the situation orally before resorting to any others means of communication. Establish accurately what the problem is and see whether the situation can be salvaged. Always try telephoning and negotiating before any further action is taken. If this is not possible, try writing one letter, or getting a solicitor to write one letter, putting your position firmly but politely and in a legal, non-hysterical manner.
This approach may yield dividends since nobody wants to go to court a negotiated out-of-court settlement is always better for all parities always include copies of any relevant correspondence or correspondence referred to in your letter. Do not include any unconnected details or long-winded explanations as to why you are now attempting to deal with the matter on a legal basis.
All this is irrelevant and will weaken your case. Simply state the problem, demonstrate your evidence and, if appropriate, state what you want to see done to resolve issue.
Similarly, after receiving a letter threatening action from a firm of solicitors, do not be immediately terrified into submission. It may all be an elaborate bluff on the part of their clients. If you have a good case, then it does not matter if you are not familiar with legal terminology-everyone understands this and al courts of law will bend over backwards to accommodate you. Any letter of response that states simply and firmly your correct and truthful position will be absolutely sufficient to answer your case