Tenant unpaid 1 month & 1/4 rent

In December My tenant only paid half the rent, she sent me a message saying she would pay the rest a week later, this was never paid. January payment was missed (because she changed the direct debit apparently) when I asked her she instantly paid half of the half of December’s missing rent & said she would catch up with the rent. Since February she’s paid the monthly amount on time but not made any effort to pay the outstanding from December & the whole month missing in January.
She’s told me she is trying to catch up. But it’s now almost June & there’s not been any additional payments for the month & 1/4 outstanding rent since January.
What should I do?
This is a openrent tenancy agreement

There is an NRLA payment plan you can use. Write to her with the plan of how it should be repaid
You can ask the guarantor to settle if she has one
You can make a court claim online through MCOL
You could do nothing
You could serve a section 21 ( just remember this could happen every Christmas)
You could speak to Gothard Rowe to mediate on your behalf. They try and find charities etc to contribute toward her debt