I have just planted a Rowan tree which is pot grown with a large rootball
(15 Ltr pot). I have staked it at a 45 degree angle, as per the
instructions. However, is it absolutely necessary to stake a 1.5 metre tree
with a large rootball? If so, how long do I have to leave the stake in?

Many thanks

Les

Yes it is necessary as the root ball will be fairly self contained until it grows in to the surrounding soil.
I would leave it a few years, ensuring the tie never gets to tight.