Tenants quite property with no notice

Other

Rent was late so went round this morning. Property is empty and appears ok (had to go to work, will go in later). They have had no contact with me and are not answering phone etc. I do not know where they have gone.

They are in arrears by one month and I was expecting March's rent, which I assume would now be due in lieu of notice?

3 main questions:
1. Can I take back control of teh property and change locks if it is obvious they have vacated and are not contactable?
2. What do I have to do legally to end the tenancy?
3. If I do not know her forwarding address, but can track down her employer, can I take action to ontain a CCJ etc for the arrears?

The tenancy was an AST, but no deposit was paid (long story!).

Thanks.