I love the little threat in the last paragraph!
The plot thickens.
This letter would certainly explain why Andrew would engage a prominent QC. It's the sort of letter that one would only respond to through one's own lawyers.
I find it interesting that the letter does not say that they want to take a sworn statement from him with a view to him being a witness in proposed proceedings against another person; it is couched in terms which suggest to me that they are gunning for him. But what proceedings are they contemplating? What has he done wrong that could result in civil proceedings against him? I thought that he could only be in the firing line for the Federal charges relating to sex trafficking. Over here any criminal proceedings would be instituted by the Crown prosecutor, not a private individual, so it would be the Crown (giggles, inappropriately) that would be seeking him out to take a statement. What is the procedure in the USA?