Help,my 2 male Jacks are normally best friends but over theast few days they have had 2 big fights,at the moment I am keeping them apart,both are 10 years old,one been here all his life,one since last June.

Has the older one any health problems, if so, he may be in pain, and does not want to play or socialise with the younger one,he may be warning the younger one to back off, and leave him alone.

Younger dog trying to dominate older dog maybe ?

yeah I'd agree with mikroc. Your younger dog is trying for top dog position, and the older one isn't willing to back down. Is it around food or toys ?
Getting the younger (or both) ball chopped off may help.

Possibly a bitch on heat in the area?

Both dogs are the same age,both been castrated,they live with a older female dog who had been done,one of the dogs seems to be the one that starts it,both have been hurt in the fight.

Perhaps one of your dogs is ill and in pain,this could be the cause of the change in his personality.It might be worth having him checked over by the vet.Good luck.