Irving quote 

Re: Authoritive statements of Irving in [Battle of Britain] article.

You have re-wrote the section as following:

Historian David Irving states that part of this reason was due to the totally inadequate production levels, with 250 single-engined and 64 twin-engined fighter aircraft produced on avarage per month during early 1941Irving 1974, p. 163., and that as a result, the number of German front line aircraft were declining, and this problem would not be improved until the spring of 1942, with a huge effort to expand production, reaching 1,200 by March/April 1942.Irving 1974, p. 142.

In fact, however, Irving does not make these statements. If you read the book  (which has been linked in free pdf format), here is what he writes on page 163:
On Milch’s desk there were already the most alarming reports not only of the gathering RAF bomber strength but of bomber production in America too. To him, air defence rested primarily on the fighter squadrons, but at present the Air Staff was calling for only  fighter aircraft to be produced a month. Fighter production had averaged  single- and  twin-engined aircraft a month during . Milch and his new planning chief von Gablenz wanted far greater numbers, and with more bombers as well  not only the  He s but  Ju s of the latest type as well.⁴⁰

etc.

Ie. Irving does not state 'totally inadequate production levels' (and even less, mentions bombers), this is the addition of one editor to the Irving book, which has been described as 'Milchs diary' (which is it not, either)