Hi Dave,
have you considered putting a slim servo in each tailplane half? This way you have short direct linkages so less chance of slop. You do need to take care which servos to use of course, but I can recomend the MKS 6125 servo. 10mm thick and on 6 volts 8Kg of torque. Theu are metal geared, double ball raced and have a metal case.