Should police be informed? - Physical Abuse on suspected women.

Hi.
It's 3am and I hear a loud volume of noise coming from outside, perhaps an argument and I decided to intervene and shout out 'Oi' so they get my attention.
I questioned their actions and in return get the typical 'What's it to you' response.
I decide to scare them off as I take note of the vehicle registration.
I thought of calling 999 but by the time they would have arrived, they would have been gone and wouldn't be classified as a 'crime in progress' .
Do I call 101?
I believe a woman has been hit by her partner because I heard a loud slap and a slight sound of pain echo to my ear.
It was awful to watch but I couldn't risk my life either.

You are right not to put yourself at risk.

There doesn't have to be a crime in progress that is going to last an hour for the police to arrive in order for you to phone 999 (not 101). You can also call if a crime is in progress and likely to become serious, if there is a threat of violence, or if someone is likely to get seriously injured (this list is not exhaustive).

I had a domestic violence situation next door to me. I was phoning 999 regularly over hearing this couple run round the house and threaten each other, police would arrive and would be told it was fine. I actually called 101 for advice as I thought it might be a problem me phoning so much, but was reassured that it wasn't.

You could phone 101 and report what you heard, and give the registration number. Even if someone in a DV situation won't press charges, the police record incidents to see if there is a pattern and offer support. But don't be afraid to use the emergency number; there's switchboard operators to do the filtering of calls.

I rang 101 because by the time Emergency services on 999 would have arrived, they'd be gone.

Doesn't matter. I phone 999 knowing my neighbours might have stopped fighting and say it's all ok when the police arrive. In your case and mine a crime was in progress and could have become serious. In yours, they could have driven off and continued fighting elsewhere. The police could have put a call out on the car. There's lots of ways it could have gone.