Hiya LG!
Sounds like he really is a handful and you getting hurt means he's too powerful.
I can really recommend Top Dog Security
http://www.topdogsecurity.co.uk/ as it's where I took my Akita. They're in Waltham Abbey, so a bit of a trek for you, but they'd more than likely keep samson for at least 3 weeks (probably longer if he's really troublesome)
These guys do allsorts of specialty training - bomb,drug, sniffer, guarding etc etc.
I had a live demo of a German Shepherd doing the 'trained to bite' an attacker's arm and was very impressed. As soon as they stopped the excercise the doggy went back to being friendly and placid - as soon as a command word was spoken, she went into 'work mode' !
The facility is large, the Police and working dogs are kept in a separate area away from the 'pets' area and there were a good 2 or 3 handlers when I was there (drop off and pickup) and what I think were teenagers from the local area assisting with the more mundane walking and behaviour repetitive training. The section where the Police and Working dogs were is quite 'frightening' as they all are behind their caged pens and growl and bark as you walk past - showing the training is working!! not all of them though, guess the sniffer dogs are more placid - but you can definitley tell these dogs have more of a working streak in them!
Give these guys a call - who knows, they may be able to calm samson down, I'm sure they've had experience with strong/difficult dogs. If not, maybe they might take Samson on? or at least know of someone who 'wants' an aggressive type dog..
Good luck !
Cheers, DtM! West London & Slough UK!