I know you can Fill a layer with a pattern but is there anyway you can have the pattern only show through your brush strokes so as to give the effect of a textured brush? I realise there are three preset textured brushes but can you edit the others to make more?

If you go to your brush settings (click the button for your brush on the top toolbar), then under Texture, make sure Pattern is checked, and then choose the texture you want on the Texture tab on the Pattern option.

For the current official version (4.3), the texture can only be used as an alpha mask, so it just controls where the paint goes and where it doesn't. If you want the texture to control the value/brightness of the paint instead of the alpha, you'll have to use the latest Nightly build. If you're using that, then on the Options tab for the Pattern option, the Texturing Mode dropdown will have an option called Lightness Map, which will do what you want.

Many thanks for the info, that's great and will check it out!

I just tried it out but whenever I select a texture and try and paint nothing happens, no paint or marks of any kind(?)

In the brush options, under Texture, there's a Strength option. Is it checked, and if so, what is the value set to? If it's unchecked, it should work fine. If it's checked, Strength has a not-so-intuitive effect where lower strength makes the whole brush get fainter, so if the strength is 0, it won't paint. If Strength is set to 0. that is probably your issue. (Note, this is only an issue in Alpha Mask mode; Strength works differently in Lightness Map mode in the Nightly version)