Tenant not paying the new rent

Rent Arrears

What do you suggest is the best way forward with this tenant.
She has been a tenant of mine for over a decade. No real problems. I decided to up the rent because it was quite considerably below the market rate. She agreed only if i modernised the bathroom/installed shower etc. This was all agreed via email conversation. Stupidly i never did a section 13. Unfortunantely the tenant has still only paid the original rent and i think i have been duped.
What's the best way forward here?
Should i send her a new section 13 from now or should i send in a section 8 notice because i have emails of agreeing to new rent...?

thanks in advance