Does anyone know how to do this?!
Yes. You would do it with one or several Mesh Envelope(s) applied to the pattern.
But not automatically, because there is nothing in Illustrator that can discern a meaning of third dimension depth and position merely from the colors in a 2D raster image. That's a function of your mind. You have to perform that function of "translating" the colors and suggested shapes of the raster image into a 2D pattern that suggests the same 3D shape.
Think about it: What purely 2D criteria would you have a program like Illustrator use to determine 3D distance in order to map something on to a 2D projection of a 3D surface like a person's face? A raster image essentially contains only two kinds of information: Position and color. So in your photo of a person's face, suppose Illustrator used some aspect of color (darkness/lightness) to determine z-distance. Let's say the darker the value the greater the depth. So in just about any person's photo, their pupils and eye lashes would be more distant than their eyeballs. A brunette's bangs would be more distant than his/her forehead or ears.
And that just considers the simplest kind of height mapping based on grayscale value. Natural photos are not straight-on shots with uniform lighting. So imagine the difficulty of a 2D program trying to discern any kind of sensible 3D contours from, say a 3/4 shot, where the distant cheekbone tucks behind the bridge of the nose.
So yes, you can use Envelope Meshes to manually distort patterns so as to suggest surface contours akin to the linework style of a president's face on a dollar bill. But Illustrator can't do it automatically for you. You can devise means by which to automate, for example, horizontal and parallel lines that get thicker/thinner according to grayscale values of a raster image (i.e.; a line-shape halftone). But you can't get those lines to automatically curve to suggest the contours of a complex compound surface.
Thank you very much for your reply I do understand that you need a mesh first - I'd like to know how to create a mesh, and then map a pattern on to it to follow the highs/lows and flow of the mesh. I have searched for tutorials but whilst I think the answer is possibly just a menu funtion I'm missing, I havent the knowledge. I intended on creating an envelope mesh using a photograph of a face underneath on a seperate layer as a template to add the detail and then add the pattern to the mesh, discarding the photograph. If you know how to do this, I'd be VERY greatful, it's driving me crazy!
I'd like to know how to create a mesh, and then map a pattern on to it...
That's backward. Draw the pattern. Then apply an Envelope Mesh to it. Then modify and adjust the Envelope to shape the pattern so that it suggests the surface shape you are trying to render.
I have searched for tutorials...
Have you read the documentation on how to work with Mesh Envelopes?
I think the answer is possibly just a menu funtion I'm missing...
No. That's what I was trying to get you to understand in my first reply. There is no magic button for what you are trying to do. You have to manually "model" the pattern by distorting it with the Mesh yourself. There's no automation for this here.
I intended on creating an envelope mesh using a photograph of a face underneath on a seperate layer...
That's the method often used to plot colors onto a Gradient Mesh. Think of that as the mesh version of tracing. That's not the method you would use to envelope distort a pattern so that the pattern suggests the shape of a surface. Gradient Meshes are different from Envelope Meshes.
What kind of pattern are you talking about using?