I saw an english mastiff once...I thought it was a horse..Classic Kelpie (Australain Working Dog usually used for herding sheep) colouration, but yours is bulkier in the head and body. So ideally a Kelpie x Lab.
I am glad my pet insurance (and my home and contents insurance) doesn't avoid any dog breeds, just as well really because nearly every other person has a cattle dog x staffy or other combinations. And the next dog I hope to get is going to be an English Mastiff.