1. Are these just to get a level of where the child is? If so, why do people prepare their children for them - does this not just inflate their score so not give the "real" level?
the tests are there to support the teachers assessment which is the level reported at the end of the year. The tests can be administered at any point in Y2 so some of these parents preparing their child could have missed the actual test by months. The results of the test won't make any difference to the level awarded ...
2. Are they one or two "tests" or are they just based on teacher assessment?
There is a maths test (or task depending on the level the child is working at) a reading comprehension test (or task) a spelling test and a long and short piece of writing which will be levelled.
3. If there's teacher assessment involved how accurate are the scores as surely there are other factors that could come into play (subjectivity about the child, teacher trying to achieve certain targets)?
The teachers assessment must be backed up by evidence in the form of work the child has produced over the whole year which will be moderated externally.
4. Do KS1 SAT scores have any bearing on anything at all eg entrance to secondary school?
The SAT scores at KS1 have no bearing on anything not even the KS1 SAT results other than to support the teacher's assessment.
5. Some private schools don't do SATs - are children in those schools missing out at all and why would some schools not do the tests?
Private schools don't have to follow the National Curriculum and these are really called KS1 National Curriculum tests (although most people refer to them as SATs).