I have also been struggling with this, and over the past 3 years of teaching the new GCSE have changed them numerous times! The advice from my line manager in the end was to follow the first year English results, and to take into consideration that as a new spec, which is much harder than the old one, that the boundaries are likely to be lower than they were on the old specification.
These are the average A*-G Grades from the past couple of exams on old specification (AQA): 85%, 76%, 65%, 54%, 44%, 34%, 25%, 15%. Therefore, I think your grade 1 is probably too high considering it is higher than the average pass on the old specification. You've got grade 4 and 5 on the average for old spec, but a 4 is considered a C grade, not a D so maybe that is too high?
The boundaries I am using are: 15%, 25%, 30%, 45%, 50%, 60%, 65%, 70%, 80%. I think my top end is probably a bit too low comparing against your grades, and considering an old A* was higher than my 9! However, these are still higher than first year English grades which is what I was advised to do.
Any feedback on my opinions is welcome, like everyone else I've just done my best to make sense of things and predict what might happen in the summer when no one actually has a clue!