text: It's probably easiest to do this yourself, as you have the specific knowledge. The editing tasks are not too difficult with the online editor:
  
  1. I would draw the remaining buildings of St Georges Mews on the Capstan Road side. You can do this as a single block (as in S side of Capstan Road) or as individual houses (as to the N)
  2.  You then need to add a single point (node in OpenStreetMap terminology) and add the following tags:
  entrance=main
  addr:housenumber=1
  addr:street=St Georges Mews
  addr:postcode=SE8 3RG
  
  This is an example of what you want to achieve  http://www.openstreetmap.org/node/3082504798. And this video may help https://www.youtube.com/watch?v=VPJz-AucqF4
  
  At the very least you could then add a link to OpenStreetMap showing the correct location when making orders: I suspect that most couriers/delivery folk rely entirely on satnavs which only show the most central address within a postcode.
  
  