Have you tried a feliway plug in?
I've seen mention of feliway a couple of tmes on these forums but otherwise I've never heard of it. What is it?
Has anything changed recently that would cause them to challenge each other?
Yes, something in the enviroment has changed. We had the utility room competely rebuilt, which was the area that the cat beds, food, water and litter tray are. Though the cats have access into more of the house, this is the area they are locked overnight (it is quite large).
Is there another cat hanging around that can sometimes cause transferred aggression?
Ooh, I don't know. There are several cats in the area, all males. At first we used to see them inour garden but I haven't as my two have gained in confidence. But there could be a new cat in the area.