Another vote for Zimo.
I've just returned to OO Gauge after a brief period in O and I've equipped my whole fleet (20ish locos) with Zimo MX600R's for the 8-Pin locos and MX638D's for the 21-Pins - as SR Man says above, £20 each.
I tried some of the Hattons 21-Pins a year or two back and didn't find them to be as good as the Lenz's I had elsewhere, but the Zimo's beat both of them in my opinion for quiet, smooth running and fine tuning on motor control.
Mike