Went all the way to CompUSA, only to find they hadn't got a 4 wire fan in the place. Told me to go to NewEgg!
So, back to mod the old fan...can't wait any longer. The labeled 12v gives 2750 rpm. A lot quieter now that I've greased the bearings, but still a bore. 5v gives 1300...still moves air, but a tad on the low side.
Now I was in for a surprise. 15 Ohms set it about right at 1890 rpm, but only a 2 watt wire-wound resistor stayed cool enough to be safe! So, a 'simple pot' would not do. The Maplins' gizmo must have had a chopper circuit in it, or been a hefty wire-wound device.