I don't know what section 21 is - but why do you have to move, is it the end of your tenancy term? if so, I've never had any sort of notice when the tenancy term expires.
sorry, just seen your earlier thread below in Related posts. I still don't understand why you have to leave if the property is being sold - couldn't you have stayed on and just transferred to the new landlord?
I havent even asked that. We have been informed that the landlord wants to sell, but nothing was mentioned about us as sitting tenants. Section 21 is an eviction notice.