Ok so first things first, they can't evict you without a formal notice of eviction.
They can do this on multiple grounds.
S.21 is for a "no fault" eviction. As long as the initial term of your tenancy is finished (usually 6 months) then they can serve a notice on you any time seeking the property back, more or less for any reason. At the moment due to Coronavirus emergency legislation they have to serve a 3 month notice.
If they serve a valid s.21 notice then after it expires, it entitles the landlord to apply for a court order.
If he gets a court order and you don't move out then he has to apply for a warrant to get the bailiffs to remove you.
This all means you can get many months beyond the expiry of the notice before you are forced to leave. Unfortunately if you drag things out in this way though you can be liable for the costs of the bailiff/ court etc.
If he serves a s.8 notice this is not non-payment of rent. The s.8 notice sets out various grounds. Ground 8 is one commonly used and this can be served if you are 2 months' or more in arrears at the date the notice is served.
Again, at the moment they are having to give 3 months notice for these and the same procedure I've outlined above applies.
So in summary, you've got at least 3 months before you have to leave, likely quite a bit longer.
I'd always advise trying to negotiate with your landlord, hopefully they will be reasonable about it all.
You might find these links useful:
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