Perennial geraniums (varying flowering times and shades of pink and blue) for lower growing ones , with taller campanulas and penstemons for later. Antirrhinums often self seed and often come back for several seasons as well. Roses do well on clay and only need dead heading, with pruning back in late winter/early spring, with a sprinkle of rose food in spring. Some crocosmias would make a contrast - they look after themselves and and only need thinning every few years. Lucifer is a brilliant scarlet one.

Most geraniums die back in winter so you can plant bulbs very close to them for colour in spribng.

All these are very easy and trouble free - (well the roses might need a bit more care but they are worth it)

Many thanks,
I will try roses I think and see how they go!