sequence of tenses

[engee30]

Dear teachers,
I'm a bit confused about when to use the sequence of tenses. I'll try to explain my problem in the context of two situations.

1. I am in London. I meet a friend of mine there. I say:
a. I didn't know you were in London.
But would it be all right if I said something like this:
b. I didn't know you are in London.?

2. I am back in my home town, which is not London. I meet another friend of mine and say to him:
a. I didn't know you had been in London.
or should it be:
b. I didn't know you were in London.
or maybe:
c. I didn't know you had been to London.

In my opinion only 1.a. and 2.a are correct.
I'm dying to know that.

[engee30]

Please the longer I wait for any help, the more confused about the problem I get. Any hints?