All good questions

Let's look at your "( probably faulty)" assumptions. As it turns out, you only missed one!

Fax_Monkey":35iyzhhr said:

1 - The Earth ambles round the Sun at 60,000 mph

I don't know the number, but sure, something like that.

2 - The Solar system belts round the galaxy at some scary speed

Indeed!

3- The Galaxy is moving through space away from universal centre at even more speed

Not quite. Yes, the universe is expanding, and our galaxy has some speed, but it's not moving away from a "universal center." The common analogy (so common, in fact, I just used it in another thread!) is of blowing up a balloon. Imagine a 2-D universe on the surface of a balloon, with little galaxies painted on. If you blow the balloon up, this 2-D universe expands - every point starts to move away from every other point. Of course, there is no center to the surface of the balloon

So neither is there a center to the universe (at least, not that we know of!).

This is important because it means that there's no stationary point (like a center) with which we can reference the Earth's velocity. And that brings us to....

4 - the speed of light ( C ) is a constant throughout the universe

Oh, right. That's also true. But the above point brings us to...

OK my confusion is this - the earth as part of our galaxy must be moving at quite some speed from universe centre.

That velocity would have a value to a stationary observer watching our galaxy wizz by in space.

Enough velocity to suffer time dilation in comparison to the observer.

So on Earth we are always under a certain amount of constant time dilation ( as part of our galaxy).

So how can we say that we know the true speed of light - when every mesurement of distance over time would only be correct 'locally' The time function of the equation would surely be different to the stationary observer?

We can't have a "stationary observer"

So yes, the Earth is moving, and as it feels acceleration from various gravitational pulls, there is indeed some associated time dilation - but it's miniscule, really, a part in billions or more. But here's the important thing - that only happens because of our accelerating motion, like that around the Sun. Just going at a fast velocity doesn't mean anything because, as Einstein famously pointed out (well, more like Galileo, but who's keeping track), who's to say that you're not at rest and the "rest" observer isn't travelling quickly?