I don't know where you bought the Rhodo, Charnley, buty if it was from a typical garden center then the roots would have been potbound in dryish peat when you unpotted it. The dry peat would not be able to take up more water easily and the plant will be dehydrated. What you should have done, if this was the case, was to soak the rootball in a bucket of water, keep kneeding it to absorb some of the water then work on the roots to loosen them up and removed as much of the old peat as possible. This is all I can think of if the plant is looking sad soon after planting.

Sadly a lot of nurseries sell rhododendrons potted up in almost pure peat. This does little or nothing to help them grow on and they need all the compost washed away from their roots and re-potting or planting in an ericacious soil mix.
In general you also need to water with rain water, though that should not be a problem at this time of year.
PM Bulba or myself if you have more concerns.

To follow on from above advice Charnley notice you live in Kent and you say rhododendron recently planted.

If you live in region near Weald of Kent , a lovely area , these are chalk uplands which rhododendron and other lime hating plants will not appreciate being planted out in.

If this the case then best in pots .