Bevans variety is a pretty tough cranesbill and stands up pretty well to heat and drought. It's not necessary to prune them back generally, but if they are looking brown and shabby there's no harm in doing so. You can pluck out the worst leaves or give it a haircut. They will put out some sporadic rebloom, but I think they are generally an earlier bloomer. Deadheading will prevent a lot of self seeding.
Don't think you're doing anything wrong, just a reflection of a period of high heat causing some disfiguring to the foliage. Good luck!
