Smoke - Fire render - Why are my renders to different to the viewport?

I have attached a screen capture that show the difference between the renders I have in Blender internal and the viewport. The viewport has fire and the render has some white bubbles. Would someone be kind enough to point me in the right direction?
Many thanks

The white bubbles are particles that are not rendered.

After really struggling, watching loads of tutorials, I eventually found a solution. I can’t tell you what the problem was but I highly recommend that if you want to do fire efffects, I suggest that you follow to the last details the explaination given by the effects creator, Miika Hämäläinen.
His site is a mine of blender info and his flame test file, available at the link above, is excellent. It got me started … and nearly fnished.
For explosions, I can also recommend PlunkoPie tutorial

and also
Blender HD

Thanks to you all