Sexually Assaulted By Client

I'm (m44) working in Denmark, for a British company and for a German client. The nature of our business is transferring technicians to/from a windfarm by boat. I have a great working relationship with our technicians and although alcohol is a no go for either of our companies employees, it happens. About 3 weeks ago there was bad weather which caused all operations to be ceased for a few days and we were all "off duty", but techmically at work as we live/work on site. The technicians invited me to their residence, from my vessel, for some drinks at around 9pm and I accepted. All was well until around 1am when the site manager came up from behind me and slapped me on the bum really hard and then grabbed me from around the back in a bear hug which prevented me from being able to escape. I'd never spoken to this guy (mid 30s) before. Note I'm 5'9" 230lbs and reasonably strong but this guy was bigger and his strength was overwhelming. Although this was taken by all to be a bit of rough play initially. It became very awkward after I told him I didn't like it and he wouldn't take no for a response. He made continued grabs towards me, overwhelming me with his strength and in front of around 15 witnesses he put his hands down my pants, tried to insert his finger into my anus and in a separate move reach into my crotch. I was wearing joggers which made his movements a lot swifter and easier than I could avoid. I wiggled and squirmed as much as possible to prevent the actions but he was too strong for me. All the while he was whispering into my ear "I know you like it", "I know you want it", "I know you want me". I told him "no", "I'm not gay", "I have a girlfriend".

After the crotch grab other technicians intervened and he was peer pressured into leaving the room to go to his bed.

The incident has left me feeling scarred and traumatised. The technicians have been playing it off since with laughter, as though it wasn't that bad and they have also told me he doesn't remember doing it. They have also informed me that he does this to other people too. I feel duty bound to take action against him as I don't feel he has any remorse and is a threat to others. I'm concerned about how to pursue this matter as I don't want to cause trouble for anyone else that was drinking when they shouldn't be. Any in house complaints may be greeted with dismissal for breaking company rules for drinking but I cannot get this situation out of my head. I feel humiliated and if I need to leave my job for this then I can accept that, but I can't let this predator continue without consequences. What can I do?