Thanks for your advice.
On the strength of this I have thought to write back to my ex-girlfriend's solicitor saying I am not willing to deal with the mother or her proxy. If the son wishes to negotiate over payments he must deal with me directly. If he prefers a negotiated settlement rather than court (the solicitor's letter says he does) he will need to make contact with me as I have no contact details for him. I know his mother will not like this and would choose not to mention the £200 right now as she will like this less and she has been very vicious in the past. I have not seen any evidence of the son knowing about his mother's approach or that the account his mother suggested paying into is his. I would welcome your comments, Kind Regards, Hugh.