Is there a way to extend the time you can talk to Siri for? If I start Siri and ask to send a message or email, it will ask who to, takes the name from me etc etc. 

When I come to saying the message I can only say a few words before being cut off. I then need to say edit to start again, or tap the message to edit manually.

However if I were to create a message without using Siri, and then tap the microphone button, I can talk to the voice recognition for longer, and manually stop it listening.

Is there a way to change the length of time Siri leaves after you have finished talking or a way to change its sensitivity to sound?
You can prevent Siri from cutting you off in the command mode by holding down the Home button until you're done speaking.  

Normally you activate Siri in the command mode by holding the Home button until Siri responds, then releasing the button and talking.  In that mode she'll cut you off pretty quickly.  Continuing to hold the Home button bypasses that limitation.