Dimension engineering sabertooth, used once cost £47.99 new about 8 weeks ago, starting to clear out surplus items i have, £20 inc postage, this is the one that will mix your motors for you, works fine.
pic on profile, please pm me if your interested.

the Sabertooth can supply two DC brushed motors with up to 10A each. Peak currents of 15A are achievable for a few seconds.

Overcurrent and thermal protection means youll never have to worry about killing the driver with accidental stalls or by hooking up too big a motor.