I was in a similar position as a child, have good recall of what I read, and made a list a couple of years ago for a friend's child in a similar position (though she was 9 I think). So - long list of possible authors, with a few remarks:
Nina Bawden, Jill Paton Walsh, Jane Gardam, Gillian Cross, Philippa Pearce, Lucy M. Boston, Diana Wynne Jones - all good writers, some serious themes but no sex or gore
Ian Serraillier (sp?), 'The Silver Sword' - this is a lovely book - I definitely read it at 7 as I remember talking about it for a Brownies badge
Michelle Magorian - these are great, though a couple would be a bit 'old' for her
Laura Ingalls Wilder books - not nearly as cheesy as the TV version would lead you to expect, I read all these quite young
Lois Lowry (some for younger children, some older, some serious, some less so - a big range)
Judy Blume is unfairly maligned I think, enjoyable for light reading - and though famous for controversial content, most of them aren't at all
William Mayne, the Swallows and Amazons stories, Heidi
Frances Hodgson Burnett (I loved 'A Little Princess' in particular at her age)
Noel Streatfeild (sic) - lots of these, very enjoyable
you mention Gerald Durrell - if she's only read 'My Family and Other Animals' (which I enjoyed very much too) there are several others; she might like Roald Dahl's autobiography 'Boy' too
my sister enjoyed Dick King-Smith books (I'm thinking animals) - though I never read them.
Then of course there are all the school stories - Enid Blyton (St. Clare's and Malory Towers and 'the naughtiest girl' ones), The Chalet School (though I didn't enjoy those till older), Antonia Forest (v. classy, but again I was older before I liked them). As a child I also really enjoyed all my mother's v. old fashioned 'nursing' stories - 'Jean Becomes a Nurse', 'Cherry Ames', the Sue Barton ones - they are horribly unPC now, and the nurse always ends up married, but I enjoyed them for the stories and always flipped boredly through the compulsory 10 pages of engagement at the end!
Hope some of that helps.