what to do now

Rent Arrears

I am currently renting a property to a tenant who has been residing for a year and 3 months in my house. For the last 3 months she has not paid rent despite constant letters, phone calls and visits to the property. I issued a section 21 at the beginning of January and she has been in touch to say she had been made redundant in December. She us illegally cohabiting with a man who I don't know of. Her references are not corresponding to contact and I don't know what route to go down now. She current owes me £1500 and I need a new tenant in asap as I am on maternity leave. I do use a letting agents who I pay 14% to manage the property but they say they have done all they can now and she had had her phone cut off.
Please can you advise me what I can do.
Thanks
Alison Bryce