Thanks everyone. I am using Astro Pixel Processor for the stacking, then post processing in Photoshop.
Had another go after some reading.
It turns out I was calibrating my camera sensors wrong. So after calibrating it the right way, I am able to get 50+ seconds with a 300mm lens.
Here is an 8 shot stack taken with 50 second exposures. I am going to have to take a lot more exposures after seeing how good just 8 looks!
Here is the cropped photo.
And here is the uncropped photo so show how much cropping is involved at 300 mm with a full frame camera.