Yes, you will get a refund if no service is provided.
14 days notice of any strikes will be provided here and on the LNWR website.
There are advance tickets at £6.50 per person each way, which will be available within the 14 day period- thus roughly halving your fare. They don't need to be booked many weeks ahead, despite whatever advice you may see to the contrary.
But full price fares can be purchased on the day at Euston- £23.40 off peak day return
Two unions are striking- the RMT (basically the guards) and ASLEF (basically the drivers).
On a RMT strike day a regular half hourly service is provided until a last return train at 1910 (7.10pm).
We should have a timetable for an ASLEF strike day (which will be more restricted) this time next week.