Plant identification please

It looks like a new guinea impatiens going on the leaf colour, a busy lizzie. They need lots of watering especially if you have thirty plus every day as we have at Heathrow so maybe too much sun. Try a couple of days in the shade and see how it goes.

It is indeed a new Guinea Impatiens [bizzy lizzie]. If it has been very well watered or even sitting in water that could also be the problem. if so let it dry out a little in shade.

if it hasn't been watered then stand it in a shallow dish of water again in the shade out of wind if possible.

it will perk up as it cools down.

if nothing like either works then check the roots for vine weevil grubs.
You can also take cuttings, they root well in water. Choose a none flowering shoot and snip it off about 3-4 " long and place in water. roots in about 2-6 weeks.

Thanks so much thornyside and seaburngirl, wonderful information as usual, lack of water seems to be most probably the reason for its lack of lustre, all advice is gratefully and graciously recieved

Looks like replanting shock. Keep the soil moist and put a pinch of fertilizer in the watering can each time you fill it up. Place it in partial sun. I expect that part of this plant will not recover so remove the parts that are permanently wilted. In the weeks to come you will see new growth on the stems. The trouble is that it is getting late in the season for a full recovery with good flower production. Good Luck!