Anon call to crimestoppers.
I only had this once, it was in London, just outside Euston station in the mid 90's
Went in and someone pushed me from behind and followed me in with a knife wanting both mine and the girls cash.
(He saw her earlier I learnt afterwards) , gave him my cash and he got distracted whilst she was getting her stash and let his guard down.
Now I am a 50-something dirty fucker, back then I was 30's, fit and a 2nd degree at Jiu-Jitsu, knife removed from hand, hand round back until it went past the point of no return (at least a break or dislocation), knee to face, thrown back out of door whilst she rang 999.
He ran off and I left when I saw the police pulled up, didn't need to get my details taken ;-)
But yeah report it - fucker