Hi,
For anyone who is interested in this, I found the solution to being able to interupt speech with speech-dispatcher. It's really quite simple, but I didn't find it in the documentation, I found it by accident. I was playing around with speech-dispatcher's spd-say function, and got side tracked for a second. When I came back I pressed enter thinking I had what I needed, but turns out it was only a - and something else, I then got a list of switches that can be used with spd-say. One of these switches is -S it's an uppercase S which silences what speech-dispatcher is currently saying. So, problem solved. I hope someone else finds this useful too.
Storm