Paying by instalments

I have a client who booked me for consulting package, I issued the invoice and contract and met her in person for the first consultation.

At the consultation I reminded her about the contract and deposit payment which she said she would do when she got home. I emailed a reminder a week later, and regularly for the next month.

Due to Christmas & New Year it was January before I picked this up again and offered the client the option to downgrade her package until she could afford the full coaching, and reduced the invoice amount due immediately.

After 2 weeks it was still unpaid, and so I contacted the client again who agreed to a 2 stage payment plan due on 18th Feb and 18th March. She sent proof of a standing order.

18th Feb has come and gone and still no attempt of payment has been made, I have chased but received no response.

What are my best options?

Thanks for your thread. I also would like to know about this.