Hiya, I'm new to Krita, and what to create sprites for a game, so I need to start with a transparent background and draw onto that.
I can do this by creating a RGB/Alpha model with a single 100% opaque raster layer. I then have to select eraser mode and fill the background to turn it transparent. then it is ready to use.
Is there a way of creating the new document with an already transparent background?

Sure, just set the background opacity to zero in the "Content" tab when creating a new image.
You can also control how the background is applied, by filling the first layer, by creating a fill layer, or by just setting the canvas color.

Thanks Lynx3d, but I really am new to all this.

If I set the background opacity to zero in the "Content" tab when creating a new image, when I write to the image nothing is shown unless I change the opacity to >0, but that fills in the background too.

I suspect this is something to do with layers. I only have one layer (the background layer) do I need more?

I really do need this explaining in words of 1 syllable!

I think that's actually a bug (that the opacity setting is used for the opacity of the layer instead of the opacity of the color). Please report a bug on bugs.kde.org.

If you don't want to delete the layer every time (you can just click on the trashcan and it will make it transparent, because it will delete the current layer, but you cannot have 0 in Krita, so it creates a new, empty one), you can create a template out of it. File -> Create Template from image... etc.

I'm not sure that it's definitely a bug because the second option, Background: As canvas colour, can give a transparent background (applied colour at x% transparency) with the layer opacity set to 100%.
Personally, I can't think of a practical use for the first option, Background: As raster layer, but I wouldn't be surprised if at least one person somewhere uses it often.
The third option, Background: As fill layer, is similar to the first but with a Fill layer instead of a paint layer.

Thanks, creating a template out of the image did the trick.
Maybe when you specify RGB/Alpha colour model, the Background colour field should allow you to specify an Alpha value, and the Background Opacity would apply to the whole layer, as it does now.