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?

Yes, you can add a pattern to the brush preset and this acts like a mask on the brush tip.
An example: https://i.imgur.com/GgtGZZA.png
Have a look at the Options tab for effects and adjustments.

Thanks for the reply!