Woo Hoo!
Crutschow - you hit it on the head.... working circuit attached. I used a BDT62C PNP darlington in your suggested configuration and it's good for a 1.6A load.
Audioguru - my op amp is LN324N.... I can certainly improve things using a better op amp as you suggested.
On1aag - The strips are by Osram, using tiny current limiting chips rather than resistors. I have a (trapezoidal PWM) system to control single colour LED strips which is practically free of conducted & radiated RF emissions but it's referenced to ground and so I can't gang 3 common anode strips - Osram wisdom, out of my control!
Roff - Maximum drain will be around 2.5A. I looked up the attached current sink (voltage controlled) and built it. It's dreadful - the feedback voltage is load dependent - I don't understand it. The load is current-limited all by itself so I'm staying in the voltage domain!!
Any refinements or further thoughts would be very much appreciated
Thank you all very much!