Thanks for the help!
The cage is what is called a 'gerbilarium', which is quite deep so they can burrow and dig. It's a large cage, about 2feet long and maybe a foot wide.
The bedding is about 9 inches deep or so, and they have a wooden house to sleep in, as well as various wooden toys to play with and chew.
I think that perhaps it is a dominance thing. I've usually had male gerbils in the past and never had a problem, but after doing a bit of looking on the internet, it seems that females can be more agressive at times.
Louise