I think the problem is, Elvis has been alone for a long time and he's used to everything his own way, a bit like my dog really!

Ok, so if the aggression isn't 'that' bad then and there's never been any blood drawn try and not to panic too much. It sounds to me like Sweet P is being bullied a bit. Is he still being a bit protective over food? Does Sweet P manage to eat in peace when they're all in the cage together or do you have to take them out to make sure Sweet P gets her fill? Does Elvis ever nip at tails?
You could try cleaning the toys and rearranging the cage, I think it's down to his lack of social skills though.