Sadly there could be several reasons for this and your tree at 40 is considered old for a Laburnum.
Laburnum trees that are over-watered (its been very wet this year) or have a fungus in the soil may have root rot. The leaves appear yellow and wilted, check for roots that are black and mushy, as healthy roots are firm and pliable.
The other cause might be some form of weedkiller? has there been any heavy spraying around you?
Sadly your tree may pose a problem now, as they are shallow rooted and if sick it could come down in a storm/strong winds.

Thank you for that, have checked roots they seem ok.
Leaves are crumbly dry although cutting into branch looks very green and healthy.
I haven`t sprayed weedkiller but can`t speak for the neighbours.
Thank you for trying to help.

If there is no obvious sign of disease. Its a waiting game to see if it recovers next year?
Looking again, I think if there had been spraying in the area other plants would show the same problem so fingers crossed.