The best and most effective way of having a positive effect on local biodiversity is to plant native local species which provide the correct flowering periods for local insects.
I havent read through all these posts, but my Degree was in Plant Biology and Ecology, and I have worked in environmantal conservation in different guises for a while now. I did a brief period in horticulture, and was consistantly frustrated by people who were interested in promoting wildlife but insisted on non-native varieties that were more attractive. The key to promoting wildlife couldnt be more anti-horticultural if it tried!
Yes this is true, there are many non-native plants that will encourage wildlife, but importantly they will encourage it in directions which stray out of the natural limits for your area. Conservation specialists do employ such strategies were appropriate but it is best off leaving this practice to those who have expertly studied ecologies to make such informed desicions.
My advise to you is to you is this, and you may not like it! Do not cut your grass! this is so important, if its too much to ask then leave a corner of your garden un cut, or as much as you can. Build a rockery, a log pile and a small water feature, then leave your garden to naturally diversify. By all means study local meadows and countryside, list the plant species you see in essentially an ecological survey, and introduce those species to your garden. Create a haven for local plant species (most importantly grasses!) on which the relevant biodiversity can grow and thrive.
If this info has been repeated previous I apologize