Send the first hardcopy letter by ordinary post and if that doesn't elicit a response, send the next one by registered post with proof of delivery.
Breaking all my own rules now for the OP in extremis, respond to the original email from the solicitor, after installing a free, limited functionality version of a tool like Mail Suite (was Mail Tracker). This tracks the delivery, opening, etc of an email and removes the deniability of receipt.
In all three communications ask the solicitor what steps need to be taken, by whom, to replace the documentation set he admits to losing, and when that will be completed. Mention the elapsed time since the original email.
Breaking all my own rules now for the OP in extremis, respond to the original email from the solicitor, after installing a free, limited functionality version of a tool like Mail Suite (was Mail Tracker). This tracks the delivery, opening, etc of an email and removes the deniability of receipt.
In all three communications ask the solicitor what steps need to be taken, by whom, to replace the documentation set he admits to losing, and when that will be completed. Mention the elapsed time since the original email.