In a case of a pre-1977 shorthold tenancy, the section 20 notice is missing?

It is unlikely that a defendant who turns up to court will then admit having been given the notice. However, it is worth a try. Otherwise, this is a fatal flaw. The only solution would be to show that the landlord is in any event entitled to possession even if it is an assured tenancy, provided you can get over any deficiencies in the pleadings.