Hi moveslikejagger:
I think Jollyoldstnickschick probably has got this right.
My DD1 had a very slow start in KS1, but with support at home has made huge improvements in KS2.
I've written elsewhere about things in various feeds - but in short our solution was to work out that DD1 learned best when doing. Practice was what was missing (school policy was little or no homework). So we've built in opportunities for her to really practice in areas she was struggling.
With maths we joined mathsfactor: www.themathsfactor.com/ - but others have recommended Kumon or Mathswhizz or Mathletics. Basically all of these offer an opportunity to work through the building blocks and practice skills - something that just wasn't happening at our school.
With reading - we've encouraged friends/ family to give books as presents and searched out various book lists for good ideas: kidsread www.kidsreads.com/lists/classic-lists.asp or use bookfinder in book trust www.booktrust.org.uk/books-and-reading/children/
With writing - our school had no writing homework at all - so we've had to get writing on the sly. Sending postcards when on holiday, thank you cards after Christmas/ birthdays/ special treats, writing off for prizes and making homemade cards for friends. I've also tried to encourage keeping a diary (which may be more appealing to a girl than a boy).
We try to keep homework to around 30 minutes a day (usually split into small chunks of time 10 minutes here, twenty minutes there), unless she's enjoying a book (sometimes can read for 1 hour). But we make sure it always gets done - no matter how busy we all are. There are nights where we moan - there are nights where she moans - but two years on - this regime of regularly reading, writing and practicing maths has yielded great results.
We were told outright that the problem wasn't the school or teaching, by the Head Teacher, the problem was 'that our daughter just wasn't that bright'. Two years on, our DD received an award for the most improved pupil at the end of term assembly and I rather got the impression the subtext was that the school realised that they had rather let my DD1 down. Fortunately we could care less what the school thinks - we've had the view it was just 'day care' for a few years now. We sincerely thought there was no reason why our DD couldn't do this work - it was just that she didn't get it and we had to keep trying until she did.
I won't say these last two years have been easy -but I will say that I'm so glad we decided to go to battle about this poor performance. It will involve you in a lot of work, but it is so worth it to be there when the light switches on - when they get multiplication by 3, or whatever.
Finally - I will say that Mumsnet advice and BBC Learning Schools/ Teacher's section: www.bbc.co.uk/schools/teachers/ have made a huge difference. I've found out about all sorts of resources. Some work - some don't - but there's so much out there now to help parents and struggling students - it really is marvellous.