We had this on Friern last year. Usually after very heavy rainfall. Several cellars in our Victorian Terrace were using a shared sewer that would flood through the service hatch in the floor as you have. We flooded twice and Thames Water eventually gave us about ?1k for the damaged items in the cellar. The problem is the storm water and the sewage all goes down the same drain - this is not allowed in new builds. Our shared sewer had a Buchan Trap just out by the pavement, and the heavy rainfall was clogging this up with leaves/twigs etc. Sewage/rain water then backfills up into the cellar - not nice. Thames Water came out about 100 times, shoved camera's in the sewer, did loads of investigation, denied it was their fault - all the usual rubbish - but eventually sorted it out by replacing all the traps on the road I think. It was a bit of a battle, but no floods since!