Thank you for all your comments.
I think I would be less worried if it was a case of her babbling away but not forming any recognizable words. But there is no babbling whatsoever.
Her brother doesn't talk for her. She just seems to inhabit this silent world where she looks on but doesn't join in.
At our house, we talk and play games and listen to nursery rhymes on a CD with me singing along, as well as reading to both children. Her attention span is short though and she will often just wander off, not because she sees something more interesting but often just to stand and watch.
I'm afraid at home, the TV is on nearly all day, and she is put to bed at night with the television on in her room. The parenting leaves a lot to be desired. For instance, the grandson (four next month) still drinks all his drinks from a bottle and has no idea how to use cutlery. He is allowed to play certificate 18 games on the X box. This is a mother who sent her first born sixty miles to stay with me for the weekend when he was thirteen days old, when she had met me only once. (Prior to me marrying my husband).
So I don't have much faith that any problem will be sorted out, hence my wish to 'tackle it' sooner rather than later.