Oh dear, Tufty did exactly this in his early days.
Attacks of the arm can, as Gill rightly says, be warded off by cushions but please be careful he doesn't then turn to your face.
Bruno is a challenged cat, from what you say it's likely he's been abused or at best neglected in the past, and does not see himself as a pet (unless he wants food!)
He's feeling really threatened, so tries to control situations with targeted violence, and if you seriously want to keep him (and this is a big IF now you know he has an owner on the Estate) he needs to be introduced like any other new cat.
This means isolation, gentle introductions and lots and lots of reassurance to your existing brood, else there could be huge repercussions for your selves and your resident cats later on.
There's a good piece in the Purrs Behaviour section for introducing new cats, have a read and see if bells start ringing.
If you take the time now to fix this, he has a good chance of integrating, otherwise set yourself up for some interesting times ahead.
It's taken me two years to get Tufty to the point he will come and go freely without attacking either of us and our residents. He was near to death when we found him, in agony with his matts, and violent as a result. Even now he will only allow cuddles when he's hungry.
It's a big ask, Paul, rehabbing this boy. I know.
And not one you've signed up for.
