Just looked this one up and there is a pruning recommendation to cut back to 2 buds near the base in late winter/early spring - did you do that or not? If you didn't then you will have less foliage near the base and should have more near the top, is that what it looks like?
Other causes of this will be the very hot weather - you say you've given plenty of water, but I'd like to know exactly what that means - how was the water given (can or hose or sprinkler) how much and how often? And growing in the ground or in pots?

They are planted in the ground and are watered with the hose pipe, the roots are covered by stones to keep cool. All of the foliage is going brown although not crunchy and dying and there does not seem to be any more at the top of the plant than the bottom. I did not prune last year as they didn't seem to put on much growth. They did the same last year and in the Spring the leaves were green unfortunately the same has happened.

So if its not a water problem, and the leaves are just dark greeny brown, but not dying off and crispy, and you have plenty of flowers, is there any new leaf growth, either browny coloured or green?