I am still giving my roses a weekly tomato feed and have been doing since March, so I wouldn't hesitate to give them a feed if they are suffering. Mine have done well on the whole during the hot weather and then the monsoon like rain, although the Wollerton Old Hall variety doesn't seem to enjoy the wet weather - flowers become badly waterlogged.
I think it is fine to give them the diluted liquid feed to support the flowers, but it is too late for the 6-monthly slow release food, I think. You don't want them to produce weak growth that gets damaged in the autumn.
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