When should I tell my employer I'm being mobilised?

As soon as possible. If you do receive a call-out order for mobilisation, it is your responsibility to make sure that your employer is aware of it. Your call-out pack may contain a letter addressed to your employer to hand to them, or this can be sent directly to your employer using JPA details.  Whatever happens, you are still responsible for making sure that your employer is aware that you have been called out, see discussing mobilisation with your employer.