Hi John de Richard in London. Herewith modification information on my gear.

Rig:
IC-251. Digital modulation applied to top of deviation pot at the junction of
        R58, R60 and R61 (just below Q13 on circuit diagram). Modulation
        voltage from standard G3RUH 1200 bps modem wasn't high enough but
        was boosted by component changes on the modem board. A higher voltage
        is required from the G3RUH 9600 bps modem; output level pot set to
        about four fifths of maximum and works well.

        PTT connection made inside the rig to the appropriate pins on the rear
        panel's accessory socket - no complications here, my TNC's PTT comes
        from a reed relay.

        No modifications done for digital output from receiver (yet).

        Have investigated modifying to provide up/down control but checks are
        incomplete. Suspect provision of this facility is possible and have
        identified (display) DRIVER IC2 pins 3/11 as the likely place to
        apply it.

        Once AO-21/Rudak starts with higher speeds I'll probably expedite the
        provision of receive output and up/down control.