In the episode The Changling Spock says that an incoming energy burst from an unidentified source is approaching at
"Warp 15".
Does this mean that the TOS warp scale was different to the cochrane scale or was this a writer's oversight like I suspect it to be?
You're confusing "transwarp" (TNG/VOY) with "multiwarp" (TOS/TAS).
In the original Star Trek, the maximum velocity the Enterprise was capable of travelling safely at was Warp 10 but this didn't mean that speeds faster were impossible to reach.
In several episodes of TOS, the Enterprise and various other items travel faster than warp 10 both with and without alien assistance;
Journey to Babel - The Enterprise encounters a ship capable of travelling 'faster than Warp 10'
Let That Be Your Last Battlefield - The Enterprise exceeds Warp 10
The Changeling - The Enterprise goes to Warp 11, Weapons travel toward them at Warp 15
By Any Other Name - The Enterprise travels at Warp 11 with no ill affects
In TNG, the scale was recalibrated to make Warp 10 essentially unreachable. Speeds in excess of (approximately) 3000 times the speed of light are described as "transwarp speeds".
They changed the warp scale in TOS/TAS so that the writers could develop a sense of limitation in TNG. Basically, think of warp speed in the original as the standard measurement of speed that the ship or vehicle could gone above the speed of light. In TNG, the warp or time-warp calculation was standardised across the federation and could measure anything above light speed. Think of the difference in measurement as being like measuring cm or inches. You could be measuring the same thing but with a different scale. Note the the ENT series apears to use the same warp scale as the TNG, which is odd. But I can’t be sure and will update this when I find out.
