Looking at the spots closer - I still suspect an insect. It must have been a very active one - I usually only get 4 or 5 spots.
(I still think if it's dust on the sensor you should be able to see it.)
Attached is your image - spots removed. (It could be done better but I hope you like it).
By the way if you want to get a better image of M42 - you need to take subs of shorter length also (maybe 10 -20 seconds) to avoid saturating the core. You then use layers and masks in photoshop to join the images of different exposures. Some people take subs of 4 different lengths to even out the difference in brightness. There are tutorials on youtube that explain how to do it.
It's actually fairly easy.
Open the image in photoshop. Make sure the image is flattened (layer flatten image).
Open the brush tool. Choose a brush size (top menu) about the size of the spots. (smaller will give a better result but it takes longer).
Move the cursor to an area next to the spot you want to remove, where the color matches the color you want the spot area to be.
Then "alt" "click" - this selects the brush color. Then click on the spot 2 or 3 times (or hold it down) until you are happy with the color match.
Do this to all the spots.
You can use this to remove blemishes in non-astro photos too.
+ Thanks for your help and others too in furthering my knowledge on imaging & processing. I'm still having trouble drizzle combining subs but that's just down to experimenting with settings and applications.