I have contacted the street and pavement repair team, and will follow up if it's not sorted urgently. The hole is on the corner of Stockwell Park Crescent and Somerville Close. It seems to be a missing drain cover or fitting for the street light. While it is relatively small, it could easily be dangerous if you don't see it and is in the middle of the pavement.
Hopefully Lambeth will sort this as a priority. If you spot any potholes or problems with the roads or pavements in your area - you can report them by calling 020 7926 9000, or email TandHCallCentre@lambeth.gov.uk