Thank you all so much for your advice and suggestions.
I think that her parents are doing all they can to alleviate the situation, but to no avail. My granddaughter certainly gets extra time with Mummy now - so much so, that she has said that she is worried that she is neglecting the little boy. (He, bless him, is so easy-going and amiable, that it doesn't seem to be affecting him.)
During half-term I had the children for two days. (And during term-time I pick them up from nursery/school two days a week, and look after them until parents finish work. On the other three days, one parent finishes work early.) On one of the half-term days, my granddaughter had a 'crying episode' and just would not be comforted. During this time, my daughter just happened to 'phone to check on the children, and granddaughter - through her tears - shouted out, "Tell Mummy I'm crying." This made me wonder if she is deliberately trying to upset her mum in some way ...? Maybe it's an attention thing. (But then, crying throughout the day at school doesn't seem to support this idea.) And she gets so much attention, anyway.
She has been questioned gently, but throughly, about bullying, friends, and has someone asked her 'to do something that's she's not happy about', etc. We're no wiser! All we get is, 'I want my Mummy'. (There could not be anyone 'interfering' with her - either at school or home - she is never out of sight of a carer - teachers, parents and grandparents.)
We are at a loss at what to do next. I suggested that Daddy takes her to school - just to change the routine - hoping it will change what has now become a habit. They are going to try that tomorrow.
Fingers crossed!