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If I were you, I would drop a letter through the letter box stating that you will sign the Deed of Assignment, once your item have been returned.
As I am sure you are aware, as soon as the Deed of Assignment has been signed, that will mean you no longer have any legal right to go into the property.
If you are unable to collect your items, via the above method, you are still legally entitled to gain access while the Tenancy is in joint names. Indeed, you are entitled to use reasonable force to gain access. However, I am sure you don't wish any trouble, and I would either ask the Letting Agent to accompany you one day to collect your items or contact the Police and state there may be a breach of the peace if you were to go alone, and ask if they will accompany you (if he say there may be an ensuing argument, they may attend with you).
I hope this assists and sets out the legal position.