Thanks for the reply. Roff, can you give a brief explanation on how this is working?
Sure. The pot develops a variable voltage on the base of Q3. Q3 is an emitter follower, making a relatively low impedance adjustable voltage source at the tops of R3 and R4. As the voltage is varied, the capacitor (C2 and C3) charging current through R3 and R4 varies, changing the frequency. Since R3 and R4 are connected to the same (variable) voltage source, the duty cycle remains at 50%.
D3 is to prevent the base-emitter junctions of Q1 and Q2 from repeatedly breaking down, which would have the potential of damaging them.
D4 is partial temperature compensation for the base-emitter junction of Q3, so the frequency will be more stable as temperature changes.
R7 prevents the voltage on R3 and R4 from going so low that the circuit ceases to oscillate. You can change R1, R2, R3, R4, and the capacitors to suit your frequency and LED current requirements.
As drawn, the frequency will change over ≈3:1 range from one end of the pot to the other.