Who is responsible for lost post?

On the 1st Oct I placed an order for an item over the phone. The next day I received an E mail stating that the goods had been dispatched on the 2nd and would take 4 - 5 working days for delivery. This took the delivery time into the P.O. strike. To date I haven't received the goods. I have contacted the company via E mail on the 13th Oct and they replied that the goods had been dispatched as promised. I have replied to them asking for them to contact the courier or P.O. depending on who they used to deliver the goods and if a signature has been obtained if they say that the goods have been delivered. If I don't receive the items who's responsibilty is it to find the undelivered items mine or the company I purchased them from. If it is the companies responsiblity how do I go about getting repacement items without having to pay again and how do I prove that I haven't received the goods which are valued at �54