As has been pointed out by Dr Ken Thompson, it's actually the long grass that is of more use to a greater diversity of wildlife than the flowers, although people seem to have a natural affinity with wild flowers and who can blame them.
Having checked out the research (alright reading a couple of books) my own approach with the grass in my orchard is to mow a couple of paths through it then cut a section of it at the beginning of June, some more at the beginning of July, then all of it at the end of summer and keep it all short from then on going into winter. Hopefully this provides the right mix of long and new short grass for the needs of the greatest variety of invertebrates. Of course I can't resist wildflowers either, many of which, like the clover, buttercups, self-heal, and wood forget-me-not need no encouragement from me as they were already there, and they are obviously of use to pollinating insects. To these I've added ox-eye daisies and bladder campion raised from seed and also sown pink campion. This year I've got some tufted vetch, field scabious and vipers bugloss on the go in the coldframe which I'll plant out later.