Ok so just to get this into some sort of order:

• D lives there and has done for 4 months
  
• D brought some people back with him (about 12 people) and had a party until 4am
  
• D's friends stayed for the weekend but were being quiet
  
• B didn't like the fact that D had friends over and told him that they had to leave (why?) which caused an argument
  
• B went out for her birthday, got drunk and when she came back with 15 friends at 2am she found herself locked out. She and her leery friends banged on D's window (at 2am) to be let in
  
• D opens the front door to 15 drunk leery loud people (who he's not met before) outside of his house and tells them to f*** off (I presume he couldn't tell B was with them)
  
• Someone in B's friend group takes offence at this and starts a fight which ends up with people being hurt
  
• Police are called but no one is arrested (probably because both sides are as bad as each other)
  
• Landlord won't evict D because there are no grounds
  

So.....legally, you should wait until D's tenancy runs out, which should hopefully be in 2 months' time. There's no real reason to get rid of him. By all means call the police when he's doing drugs. Or if it's that bad, move out yourself.

By midday D’s friends were still in the house, now in his room and quieter but they were still going through living room to get to kitchen and this annoyed B, she went to D to ask him to get his friends to leave.

Why should his friends leave just because you don't want them there?

they had gone back home with about a group of 15ish people to have some drinks back at the house (this was about 2am)

And I guess B and her 15 friends would have been as quiet as church mice? Why is B allowed to have 15 friends over at 2am but D isn't allowed 12 at 11pm/3am?

TBH you sound as bad as each other. It's his home too and yes, he shouldn't be partying loudly until 4am but this seems to be the first time he's done that in 4 months.