For a lovely show of blue from April to July you could plant Gentian............it's low growing and spreads slowly, and bees love it. You don't have to dead head it either so it's low maintenance.
Lupins, once established, just do their own thing at the back of the beds and keep insects interested for weeks, too.
There are all different colours of perennial Geraniums which flower well across the spring/summer period, and hover flies seem to like them.
Honeysuckle, mints, Cotoneaster.............all loved by bees and flies........red/white/blackcurrants.........and, of course, Buddlea (sp) if you have the room.
Some of the scrambling roses are easy enough....they only need a prune at the back end of the year to get them away, trouble free, in the spring.
Hawthorn, too, in the hedge helps lots of insects in spring and their flowers are lovely, if short-lived.
Alpine strawberries..............................