Stephen isn’t a Platt (a prat maybe) but he is Audrey’s son from when she was 16. Audrey isn’t a Platt, that was Gail’s former married name
Stephen is meant to be Canadian.
Leo discovered that Stephen was trying to con money out of Audrey and he confronted Stephen with it on the balcony of the factory. They got in a scuffle and Stephen chucked him over the balcony and Leo landed in the factory’s wheelie bin. Dead
Stephen moved the body to his van and then buried it somewhere. Jenny & Leo were meant to be emigrating to Canada that day so all his bags were packed. Stephen snuck back to the Rovers, removed his cases and his passport and burned them so that Jenny thinks that Leo went to Canada without her