A very angry Val here... what horrible, horrible owners. 
I'll do my best, JollyTired, if you'll let me. And please, don't worry, I don't bite too often. :)
Firstly I must ask you to beg and beg the owners to hold on as it won't be easy but it CAN be done although it will take time. I'm bloody minded and determined but I'm not a miracle worker and an older dog with alleged unpredictability issues isn't going to be easy to place. This sort of thing is, well, it's what I do, as an independent rescuer I find places in no kill rescues across the UK for dogs like Rupert and for pound dogs who would otherwise be killed.
Next, please can you pm me to tell me whereabouts Rupert is at the moment, what he's like with other dogs, with cats, with adults and as much as possible about his unpredictability.
I'll also need to know if he is neutered and if he's fully up to date with his vaccinations.
I'll explain more and provide you with references so you can check out who I am and will give you my contact details too.
HTH. :)