As long as your pass is activated before the stated date you can travel on any train in the network. All you need to do for trains without mandatory reservations is add them to your trip in the app and operate the slider to add them to your pass. This can be changed at any time by reversing the slider to remove the original train and then adding the new train to your trip and transferring to your travel day.

Many thanks yorkie, much appreciated.  Can i  just clarify then that activating railcard is enough, I don't have to add journey to pasd or physically travel by 11th? And I could decide to travel earlier than 11th too? Thanks.

Carole 

You need to activate the pass with a start date no later than the 11th of April. You can then travel whenever you like on or after the start date of the validity of the pass. 

Thanks Anna. So I could also change start date to earlier than 11th or nor? Sorry for being so needy! 

Carole 

You can indeed start at any time up to the latest activation date, but not later. Obviously once you start the clock you have the specified duration from that date to complete your travels.

Just to be clear you do not have to start your travels on the day you activate but cannot travel after midnight on the final day of validity.

Many thanks Anna - very helpful and clear!

Carole