Answer: No, she has not met Lennon before, and she's unsure if it is really him. The whole conversation is written so that their bantering does not make any real sense, and it humorously depicts how famous people are sometimes perceived by the non-famous. Basically, the woman doesn't know what she is talking about but wants to sound like she does.
Question: After the scene where John has his "you look just like him" conversation in the corridor with that woman, George goes to the "canteen and production offices" and sits down in some sore of reception. The receptionist says, "Oh there you are". Could somebody please tell me what the hell goes on in the scene that follows, where George is given those shirts? Did he just stumble in accidentally?
Answer: Yes, George just happened to wander into the office by accident. He is then mistaken for someone else they were expecting who was to give them his opinion as a "typical young consumer" about the latest trends and fashions they are test marketing. These "experts" are actually so out of tune with what is actually happening in the current youth culture that they do not even recognize someone as famous as George Harrison, who criticizes their products (the men's shirts) as being "grotty" and ridicules their model spokesperson, who he calls, "a well known drag." That is when George is thrown out.
Answer: John is just messing with her. She recognizes him as being one of the famous Beatles, but he tells her that he's "not" John Lennon, to which she responds that "you look just like him." He finally convinces her that he is not John Lennon, and she tells him, "Actually, you don't look like him at all," to which the now insulted John puts on his hat and leaves.