I both rent and have several IP's.
I can empathise with your tenants. They are happy renting where they are. I presume they are keeping up their side of the bargain by paying whatever rent you have asked on time. It is a pain in the **** to move, its time consuming and costly and in parts of some capital cities difficult to find a suitable place to rent within 2 months.
Sometimes tenants (like landlords) have other things going on in their lives (planned holidays, exams, family crises, changing jobs, kids starting school etc). Having to find something suitable to rent in 2 months out of the blue may be difficult for them. They will also need to find 4 weeks bond for their new rental and may have to pay this before the bond on their current rental is released. Again they would not have been planning for this.
Empathise with their situation, but not condone their attitude. This is one of the drawbacks of renting. It is not your house, you may be inconvienienced if then LL changes their plans.