You need to go through the procedure & get a Court order. The Court notice will be served on her at the address you have (i.e. the place she is renting from you). That will be legal even though she will not get it if she has moved out.
If it is an assured shorthold tenancy & she is more than 2 months in arrears with the rent, then you can get a Court order without deifficulty. It normally gives the tenant 28 days to move out. After that time, if she has not moved out you have to go back to Court for "enforcement" - i.e. bailiffs to go & evict her.
You can get more advice from CAB, or possibly by ringing Shelter helpline (although they generally help tenants rather than landlords).