1. A method of generating texturing output data using a texture in a graphics processing system in which textures to be used by the graphics processing system can be stored externally to the graphics processing system and pages of the textures then loaded into a local memory of the graphics processing system for use by the graphics processing system to generate texturing output data, the method comprising: when the texturing operation of the graphics processing system is to use a texture that is stored externally to the graphics processing system at a particular mipmap level to generate texturing output data, the graphics processing system: determining if the page of the texture at the particular mipmap level and encompassing the region of the texture to be used to generate the texturing output data is available in the local memory of the graphics processing system; and when the required page of the texture at the particular mipmap level is not available in the local memory of the graphics processing system, determining if a page of the texture at a less detailed mipmap level than the particular mipmap level and that encompasses the required region of the texture to be used to generate the texturing output data is available in the local memory of the graphics processing system, and, when a page of the texture that encompasses the required region of the texture to be used to generate the texturing output data at a less detailed mipmap level than the particular mipmap level is available in the local memory of the graphics processing system, using the available page of the texture that is at a less detailed mipmap level than the particular mipmap level to generate the texturing output data the method further comprising: storing entries in a page table corresponding to each page of the texture; and associating metadata indicating a next lower detail mipmap level to try with texture pages that are not present in the local memory of the graphics processing system, and storing said metadata using data fields in the page table that are used for another purpose for pages that are present in the local memory and when a required page of a texture at a given mipmap level is found not to be available in the local memory of the graphics processing system, using the metadata indicating the next lower detail mipmap level to try associated with the page of the texture that is not present in the local memory to determine the next lower detailed mipmap level for which it should be determined whether a page that encompasses the required region of the texture to be used to generate the texturing output data is available in the local memory of the graphics processing system.