This is an old style, traditional aluminium sliding door in Kennington SE17. It has been dragging along the bottom and scraping the rib of the metal track. Thankfully, it has not really done any damage to the track itself, but the wheels will need replacing. There are two sets of Wheels which will need to be taken out from underneath and replaced which means that the door will have to be removed and the panels at the bottom removed in order to replace the wheels. Once the wheels on the sliding door have been replaced and the panel, put back the door then be replaced and realigned so that the Locking bolts align with the bolt points on the frame.