Seriously? If you are at home and hear screaming and crashing for the first time next door would you call the police?
Of course. It wasn't just screaming and crashing you described though....
I heard what can only be described as taunting, then him shouting very very loudly, then thumps and smashing noises then crying.
What you have done is alerted the abuser, given them a warning. This will serve two purposes. 1. They will be more careful in future and 2. They will probably take it out on the victim.
Please could I just have a sensible range of optionins on what I should do to keep her safe and me safe?
Calling the police WAS/IS the sensible option. That's what people have suggested, what else do you think we can suggest?
Also, why do you feel like you need to be kept 'safe'?
but I also have a duty to my friend whose house it is, don't I
I don't see how this is a major factor. Ok, tell your friend, but more of a courtesy, not a first port of call.
FFS it's not my responsibility is it!!
No, it's not. But this isn't about responsibility, because the abuser will always be responsible. You are however in a position to cause intervention which could potentially save a life. But you know, it's up to you.
To be clear I'm not sure what I heard - I don't know it was physical abuse.
You were quite clear in the OP