I have been informed that pre existing 1995 incapacity claimants will be taxed when they are told they are being transferred to ESA by the DWP.
This will probally be done by sending you an ESA50 to fill in.
On my understanding that is when its starts to become liable for income tax.
So for instance if you receive information that you are being transferred to ESA say in January 2012 that is the start point for you being deducted tax from your ESA.