Dear Charles,
Many thanks for your letter and the formal invitation you enclose. It is very nice of you to offer to meet our expenses while we are in Britain. I gratefully accept it on the assumption that we shall have the pleasure of receiving you here as the return part of the exchange visit.
We can discuss all the points with you personally or by letter should you wish to come over before we go. I suppose this would be a very good idea as the dates of our visits are still vague. Anyhow, I want you to understand that we shall be delighted to have you here with us any time you choose to come.
Once again many thanks and best wishes from my wife and myself.
Yours sincerely,
Nikolay