(England) Landlord demanding more money after section 21

I have been renting a property for a year now. In December we were informed the landlord was happy to renew at the same price but on rolling monthly. Last week out of nowhere we got a section 21 through the door. Monday a member of the property management team told us that she had convinced the landlord to cancel the section 21. However, he now wants an extra 4.5% on the rent. Is it legal to hand a tenant a section 21 but agree to remove it of they pay more. To me it feels like blackmail.

Any help you can give me would be amazing! Thanks xxxx