I have just planted a robina tree having dug out building materials on a new site. I dug a hole two times size of root ball and filled with multi purpose compost and a feed of blood and bone. Planted three weeks ago the leaves are now turning white, but

If you have simply dug a hole in a load of builders rubble and filled it with compost the tree is going to have a hard time. Have you been watering it regularly? What is in the rubble?

Not much wrong with the way you planted it, by the sound of things. Whitening of leaves usually means sudden exposure to cold - have you planted the tree in a windy, open area? It might be just culture shock from being in a pot in a sheltered spot and then being placed in an exposed position. More detail of its environment would be good.

If you've actually planted the roots in a dug hole and then just filled with multipurpose compost then its not the best way.
The compost should be well mixed with the soil from the hole and i imagine the soil is clay so this will break the clay up and encourage the roots to venture beyound the hole.
The roots are quite likely to stay with in the newly dug compost filled hole as its easier than venturing into the soil.
You want the tree to root into the soil rather than sitting there.
If it is clay then it would quite likely to hold water in winter as its basically a clay lined hole in the ground. Also not good for the tree