I visited the flight deck of an easyjet boeing 737 on a night flight from Luton to Nice this last March. Shortly after boarding the plane I asked a member of the cabin crew if it would be possible to visit the flight deck, they said they would ask the captain. Later on in the flight the attendant (who I had asked earlier) and told me it was ok.
The captain was very pleasant and, as I showed an interest in aviation and some knowledge of the subject, he showed me their flight charts illustrating the waypoints we were flying. I asked about the approach into Nice and he said as the conditions were clear that they would be doing a visual approach, with the first officer performing the landing.
I returned to my seat as things were getting busy, we were starting our descent. The captain did mention that it was a shame I hadn't made my request earlier and I could have then stayed longer.
I have visited the flight decks of the following aircraft in flight :-
British airways A320
British Airways Concorde
Easyjet boeing 737-300
I have always asked the cabin crew near to the start of the flight, especially if it is very short. I have never flown jump seat but would love to and I think the comment in an earlier post of writing a letter, is a good one. I will try this on a future flight.
Hope this has been of interest,