You must use a current limiting resistor for every COB LED.
About 8.2ohms, but 10ohm if you want to be on the safe side.
They use ~0.5watt, but I would use 2 or 3watt cement resistors for easy mounting on the heatsink.
With these resistors values, LED imbalance is not relevant anymore.
With 9 LEDs and 8.2ohm resistors you are just over the current capability of the supply.
With 10ohm resistors, just under.
So I gave this a go using 10ohm 1/4W resistors (as it's what I had to hand) and it seemed to work well but I didn't have them on for too long. They didn't get hot at all even without a heatsinnk.
Since my first post I've been reading up on powering LEDs and now understand it a bit better. Therefore I'd like to use a proper constant current supply but I'd still like to make my own rather than buy one as it's more enjoyable and I like the learning process.
I came across this (link below) as an option.
With reference to figure 4...I understand each string of LEDs has its own transistor so that the string will turn off should an LED fail and it won't damage the LEDs in the other strings. But can the current draw be uneven across the LEDs in different strings or even in the same string?
As I have 9 11.5-12V LEDs and a 12V power supply would I be better using one linear voltage regulator for each LED? Basically replicating the circuit in figure 1 9 times but linking them all to the same power supply?