rent 23 days late

Rent Arrears

This tenant has been paying late 15 times out of 22 months so far. Usually a couple of days late no major concern. But the rent was 18 days late last month. This month rent is 23days overdue and shes not paid. Chased up a few times she says she's been ill and money is tight etc and promises to pay with credit card advance cash but still not paid me.
Rent is due on 5th every month.
I go away from mid Jan till mid Feb. So if I serve her section 21 on 5 Dec I will be away when it is ending on 5 Feb. Ive never done section 8 and have read it is a very stressful way.
Advice please. Whats the best thing to do now?