It sounds as if you really do need some help - i remember reading a few years ago about a dog from similar circumstances, called a "Moscow Red Star" or "Red Star"? Only this was trained to be highly aggressive to both dogs and people and only one person could ever get near him!
I would suggest you have a couple of choices; first of all, to get control, introduce him to a headcollar (if you haven't already) as this will enable you to control his head and break eye contact. Use also a double lead, attached to both the headcollar and the normal collar. If you can't find this equipment, take a look at kumfi.com - you can ring George who may be able to advise what is best in these circumstances.
There is one trainer who holds "growl" classes in Middlesex, but I'm not sure if your dog would be suitable, but you could try. If you take a look at apdt.co.uk and take a look at the county list, it's Chris under Middlesex.
I don't know this trainer personally so I'm not sure if they can help, but if not they may know "someone who can"

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Another option is to maybe take your big fella to the vet and ask for referral to a reputable behaviourist who deals specifically with aggression. Make sure the b. is a member of the UKRCB or APBC and ask for refereences if at all posible, as this person really must know what they are doing or they will make matters worse!!
It may even be a case of help with just managing your Rottie, as he has ben encouraged to attack and has been attacked, many times, and you have to keep other dogs' safety in mind too

Hope this helps a bit anyway. Do let us know how you get on.
lindsay