Do the following symptoms describe your Rocket?
The plants wilting in the heat last week and the week before
The top foliage revived after the rain
Older (lower) foliage badly yellowed, some with brown margins, and some dying off
Lower parts of some stems rather pale or yellowish
Plants generally looking thoroughly tired
If so, then provided that you can not see any dark round patches on the leaves or stems, your Rocket are almost certainly suffering from the effects of the recent heatwave and the dry weather which preceded last weekend`s rain. Mine are exactly the same and so are many more that I have seen in the last few days. Quite simply, the plants are starting to die off a little earlier than usual, but they should struggle on for a bit longer yet if we don`t get another heatwave and drought.
However, if you do find round dark brownish, purplish or blackish patches on leaves or stems, then it is almost certainly blight