As Q2 heats up the current limit does drop at 2mV per degree, as this is using the Vbe as a reference. It is somewhat non linear with supply voltage, but is good enough to drive a LED load to within about 2% if you use a 1% resistor as a shunt/sense resistor. Q3 should be another NPN so that you are sure to saturate the switch and turn off the mosfet fast, and it will be better to reduce the value of R1 so that you have around 1mA flowing to provide the pullup so that you can get fast turn on of the current source. Keep lead lengths short around Q1, Q2 and the sense resistors, and keep the gate leads short as well. A 100-220R series resistor on the gate will help damp oscillations on the mosfet during turn on as well, along with having a large value capacitor on the supply rails to decouple them.