Effect of metal on electromagnetic field / Coil performance
1) What would happen if i replaced my plastic former / core with a
  stainless steel one? What would this do to the field, I am guess it
  would reduce it but would it also change its shape?


Eddy currents would be induced in the SS former and probably render a colpitts oscillator to a state of uselessness. It will act like a shorted turn in your coil.


  2) What would happen if I introduced a large piece of stainless steel
  into the proximity of the coil (think mounting bracket etc..) how
  would this effect the field strength and shape?


Providing the SS does not sap too much of the magnetic flux generated it should still work but, the SS would mask the field in its vicinity and make the oscillator's ability to discriminate various metals significantly harder.


  Is there a good reference source for this subject?


I designed food and pharmaceutical metal detectors for many years and I never found anything really useful on my subject. Discrimination between various metals was important and operating frequency can switch things around a lot. Regarding Q2, the type of search head we used was fully enclosed in SS and this was useful for shaping the field for where it was intended to be - don't misunderstand, you can't focus a mag field!