The problem is the number of 'ifs'. Ultimately, if someone (anyone) tried to evict you using the s21 notice then you would attempt to defend based on the info here. It would be up to a judge to decide who is the landlord. However, even if the claim was dismissed, with section 21 it would be a simple matter to correct the errors the court identified and then re-apply. Eventually they would get a possession order.
The practical solution is probably to consider the new landlords proposal to sign a new agreement, giving everybody a degree of security and clarity.