How''s about you reply to the letter with a comprehensive list of all the belongings ... give dates, again, when he can collect and/or advise you which belongings you can dispose of.
Set a time frame, say 2 months, that is more than reasonable. Point out how long it is already.
Lastly find out what storage places charge and say that after that date you will be charging £XX a week for storage as per the going rate. You can''t risk moving it into paid storage and then walk away as they will come after you for non payment rather than him.
Don''t be threatened by his solicitor and let them know that - they love to worry us so don''t let them.
Or, and thinking rather on my feet and I''m not in a nice mood today ... break something ...accidentally ... may make him want other important items out from your clumsy hands!