Spoke to Phil to see what he had to say and this was his reply...

"Hi Karl,
hmm, his Maths doesnt hold up, as you dont get 12V across the resistors, you get around 6, and it will be PWM at around 85% duty cycle max, but I do agree 10W is close to the limit.
Phil"

So perhaps the 10W ones are ok, and I have just spent £15 on those other ones unnecessarily!
I shouldnt be so fucking impatient!! lol
But a bit more headroom is always handy I supose!