1. I haven't seen her since i was in london
  => The last time I saw her when i was in london.
 =>I last saw her when i was in london
2.The last time she wrote to me was when i was in London
=>She last wrote to me when i was in London .
=>She hasn't written to me since i was in London .
3. It's 3 years since i last met her 
=> i last met her 3 years ago.
 => i haven't met her for 3 years.
4. i last saw her when she returned to her hometown 
=>i haven't seen her since she returned to her hometown.

1.

The last time I saw her when i was in london.
I last saw her when i was in london
2.
She last wrote to me when i was in London .
She hasn't written to me since i was in London .
3.
 I last met her 3 years ago.
 I haven't met her for 3 years.
4.I haven't seen her since she returned to her hometown.