Job description

Last Tuesday i noticed a leak coming from the light fitting in the dining room. The shower drain is directly above the light above the light fitting so naturally I thought this was the problem and I re sealed the shower.

The leak got worse and the plaster board ceiling was probably beyond repair so i just pulled down a hole around 2 ft by 3ft to have a good look at what was going on. The leak appears to be what i think is the soil pipe. Around 10 inches above the line of the ceiling there is another layer of wood below whats probably the floor boards. The leak looks to be coming from where the soil pipe goes through this wood and it looks like there may be a join in the pipe there. There isnt much if any space for movement in the pipe where it goes through this wood and when we flush the toilet you can here a banging or knocking noise so im not sure if movement over time has broken or seal or something. The problem is in an extension which is only around 8 years old i have recently bought the property so im not sure if this is a longer standing problem that has been well covered up or just started.