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Adrian
Always start with your lowest power and build up from there to find the best image.
Also waiting until it is reasonably high in the sky will help.
The black dot you were seeing is actually the secondary mirror image. If you find a reasonably bright star in your scope and defocus the same black spot will appear, nothing wrong with the scope, just another normal property of newtonians.
One other thing, your mentioned using a 1.5x erecting eyepiece. 2 comments on these, they tend to be very cheap accessories, the one I had was very poorly made, and are usually supplied so a scope can be used for terrestrial targets, as a result are designed to be used with low power and not necessarily focussing at infinity.
Malcolm
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