Cool story bro. Also:
Your brother - occasioning actual bodily harm contrary to s47 Offences Against the Person Act 1861, common assault, blackmail contrary to Theft Act 1968 s21, conspiracy to blackmail contrary to Criminal Law Act 1977 s1.
You and other mates - accessory to occasioning actual bodily harm, accessory to common assault, blackmail contrary to Theft Act 1968 s21, conspiracy to blackmail contrary to Criminal Law Act 1977 s1.
Serious Crime Act 2007 s44 - Intentionally encouraging or assisting an offence
(1)A person commits an offence if—
(a)he does an act capable of encouraging or assisting the commission of an offence; and
(b)he intends to encourage or assist its commission.
OP - unfortunately there isn't anything that you can legally do here. Private sale is almost always buyer beware. You do get some rights, but they are very limited. Misrepresentation does not cover situations where the fault appeared after the sale.
You can try to bullshit him, threaten to sue etc., but it will be pure bluff. Might work though, you never know.
Turning up with some other people to try and get your money back is not an offence is it ?
And are you saying you can threaten them though ? ...........