To ponder legal action for being slapped on the arse

I would think a civil claim would likely be costly and unlike to succeed.

I wouldn't hold out much hope for a successful criminal prosecution either (though he has committed a criminal offence), but if you report it to the police then hopefully they'll at least interview him under caution and maybe the shock of that will be punishment enough for him.

He's a twat, he sexually assaulted you.
But, if you did pursue it, will it mean you can no longer work, and in the end, end up in a worse position overall? It sounds like you dealt with it rather well.

This! Do the posters who think OP is overreacting think a slap in the face would be ok too? Why does adding an element of sexual assault somehow lessen what he did?

As others have said, I'd recommend thinking through what outcome you want and base your actions on that. But remember you have been assaulted, the fact it was your arse does NOT make it less serious. Massive well done for the way you handled it in the moment too, despite the shock you must have felt.

Don’t let this go op. It’s very clearly sexual assault of some kind and happening in front of so many witnesses too.

How bloody dare he! I would consider contacting the police, it’s outrageous.

Oh look, victim blaming with a nice bow wrapped around it.

Someone who witnessed this should have taken further action and reported it. At my company this would be immediate suspension.

What’s an unregulated industry? A business may be unregulated, estate agents spring to mind, but they still need to provide a safe workplace for their employees and that covers contractors.
Someone is responsible for your work environment. Complain to them if you want to take it further.

You aren't over reacting all at. This was assault and an apology doesn't cut it. In my workplace an act like that would lead to disciplinary action. I understand it's more complicated in your situation but I agree with others who said document it all, contact the police and take it from there.

You can report it online op via 101 website if you feel this is an option you’d like to take. But call ACAS as well

You dealt with it brilliantly but you are also left with it. Easy to say in hindsight, but I wonder if the time to take action was there and then ie to not have gone ahead and carried out the work. If you had fallen and broken an ankle you would have received a lot of fuss and care and wouldn't have been expected to carry on and do the task. By carrying on after the apology, something gets covered over as if the apology has now dealt with it and it is filed away as a slight miscommunication or similar and you are also left on your own with it after the event. Making a huge fuss at the time eg you don't accept the apology, you demand he leaves the room, you ask to speak with the manager, you demand a cup of sweet tea and a sit down, you say you don't think you can carry on with the job - makes it harder for everyone present to smooth it over. I think it is this bit - everyone acting as if nothing too bad happened- is what leaves you feeling you want to take further action but I do think it's harder to do so now it has passed. I would make a record of it and then process your feelings. I don't think you will get what you need from the company at this stage.

Initially I was thinking the same as you. But as a man I would never do that to a female work colleague. I wouldn't even touch a male work colleague that clearly I don't know well (she's contracting to a company so I assume it's not a long term working relationship). In my job with a funeral directors I might brush off a colleagues shoulder where dust from the coffin has got on it or sort out a collar as they would for me.

There is no way anyone irrespective of sex, relationship, friendship should be carring out such a sexually aggressive act.

OP I would speak specifically to senior management and express your upset.

This is work. Not a carry on film

You are not overreacting. I would have slapped his face but I am of a generation where this happened a lot and that's how women dealt with it. Report him to the police.
I don't understand why other people apologised. How was it their fault?

I disagree that he did it for a sexual thrill. I believe men commit sexual violence to degrade and humiliate. It’s a way of asserting dominance when their ability won’t do - you may be competent and impressive, but I am a man so I get to reduce you to a bit of meat.

OP - what are your actual options work-wise? In reality, if you kick up a stink what is likely to happen? Will it just lead to you not getting work?

I think you've had good advice above from @DefyingDepravity but I want to add my voice to those saying you are NOT OVER REACTING so you don't think you are going mad thanks to the male violence minimisers on this thread. We have a right to do our jobs without being assaulted.

I'm really sorry this happened to you. I'm angry on your behalf and if you're not able to feel angry but shaken and upset please take a hug. And then have some of my anger for motivation to eviscerate this fucker as effectively as possible. 💐

He won't do it again to OP, who he'll have categorised as uptight with no sense of humour. It won't teach him that slapping a woman is wrong and has consequences.

Take it as far as you can. Police report and see a solicitor if the response isn't satisfactory. You may get the police to have a word, or get a solicitors letter that will show him you consider it serious.
This man needs to really understand that this isn't acceptable. He likely makes a habit of it and apologises if he has to the women 'with no sense of humour'. He needs to understand there are women out there who will make a big fuss so he'll think twice next time.

It's a sexual assault.
It's not banter.
He did it for a sexual thrill.
You have witnesses, so you could report it to the person who hired him if you wanted to. If I hired him I would want to know.

I agree. You let him have it verbally, and he apologised and probably felt incredibly stupid. I doubt he’d do it again.

Report it to the police, it’s classed as sexual assault. There’s even a government scheme that would pay you compensation as a victim for this.

That’s a lot of words to say you are fine with people hitting each other at work.

Agree with this. I think you’re overreacting. A swift “don’t fucking do that again” would have done the job.

This.

For those saying go to the police: bear in mind that this can have a range of outcomes. It is possible that the police will handle it really well. But I also know people who have been further traumatised by an inadequate police response.