bexamy, I'm not in Kent which I believe is very competitive, but a year ago I didn't think my daughter had much chance. She's never been top of the class. She looked round the gs in July and agreed to do a few practice papers, and somehow passed and is going to gs.
Her school were amazed she'd passed (she is quiet so I don't feel they see her true colours)
I would try not to listen to other parents, your daughter is obviously quite bright, if she wants to do it, I would let her. My daughter's levels weren't great, like yours she was on a 5C for maths and 4A for writing. Sorry to hear she had time off for illness, I suppose that means you could appeal if she only missed by a few marks.
I would let her try it but try not to let her worry or get stressed. Give her a few practice papers (I'm a fan of Bond 10 minutes but don't know if that's appropriate for Kent) and see how she does.
Nobody expected my daughter to pass, lots of other mums assumed she was going to the comp because I never brag about her, and were most surprised when she passed. If I were you, I'd let her sit it. You just never know. Good luck with whatever you decide.