Check South Western Rail lost property office

I left an item on the train in Southampton on Tuesday 15th January. I've registered it on the SWR lost property website but haven't heard back; have read online that the best thing to do is visit the lost property office, but I don't live in London! I'm looking for someone to go there, ask about the item, retrieve it if it's there, and post it back to me. It's a handmade child's backpack, made of blue and blue-and-white check material, with the cartoon character Lightning McQueen and the name "Ben" stitched on it. It contains a change of clothes in size 3-4.