When it is dormant is your best chance. (It is wise to check the eventual height and spread of any tree or shrub you plant! )
Dig a large planting hole for its new position before moving it and check that this position is suitable. Rowans can make big trees and you will not be able to move it again. Dig it with as large a rootball as you possibly can and don't do it when the ground is dry. Remember the roots will cover as wide an area as the branches. If it survives it will need a lot of water all year next season as its roots recover.

as soon as it has dropped its leaves providing the garden isn't frosted. failing that march/april before leaf break as the ground warms up.
advice re planting as above.

Thanks for those two replies, that's very helpful.