  We present predictions for the numbers of ultra--cool dwarfs in the Galactic
disk population that could be detected by the WFCAM/UKIDSS Large Area Survey
and Ultra Deep Survey. Simulated samples of objects are created with masses and
ages drawn from different mass functions and birthrates. Each object is then
given absolute magnitudes in different passbands based on empirically derived
bolometric correction vs. effective temperature relationships (or model
predictions for Y dwarfs). These are then combined with simulated space
positions, velocities and photometric errors to yield observables such as
apparent magnitudes and proper motions. Such observables are then passed
through the survey selection mechanism to yield histograms in colour. This
technique also produces predictions for the proper motion histograms for
ultra--cool dwarfs and estimated numbers for the as yet undetected Y dwarfs.
Finally it is shown that these techniques could be used to constrain the ultra
low--mass mass function and birthrate of the Galactic disk population
