Is the brick wall South facing? If so it'll be like a storage radiator so the heat will come off it at night & the plant won't get any relief as it would at night in it's natural environment

Mine has done the same. These little alpine dianthus tend to be short-lived. I would take the live shoots (pips) and pot them up in some open compost. I do this a matter of course to replace the original plant every two or three years.

No wall is east facing. Will take some cuttings and hope for the best.

Just noticed the raised bed is made of concrete. Is it old or fairly recent because if newish the roots might have got burned from contact.

Check the container for an ants nest. They seem to love going under Dianthus and their acid kills the plant in quick order.

Will do.

Mine seems to have died too. Tried to revive it with plenty of water but it still looks poorly. I'm hoping it will recover now we've had a lot of rain.

Have taken some cuttings and cut out dead bits. Will be watching if it grows back.

Update; couldn’t resist getting another dianthus at my favourite garden centre. Was planning to plant it today but today is going to be pooring with rain all day so maybe not. Took a friend new to the area and she loved the plant nursery as much as I do! Spent a lovely day with another garden/ plant lover and getting to know a new friend, recommending places to go in the area.