alder buckthorn problem

Hi all
I wonder if anyone can give me any advice? I have a small alder buckthorn. it was just like a big twig when I got it, but it flourished in a small pot in my kitchen and soon had lots of leaves.

Last week I finally got round to planting it out in the garden. It seemed ok to start with but now the leaves have all gone yellow/brown and started curling up!

Our soil is clay-y, but we mixed it with sand and a load of topsoil and some compost! I've been watering it well.
at first I thought it was because of all the sun we've had lately, so watered it extra, but now i am worried that maybe it is too much water??? so not sure whether to keep giving it more or not???

any help would be appreciated as I'm not very green-fingered unfortunately. thanks