I am getting a mechanical chattering noise on idling that rises and falls, only audible from inside the cab. Slight throttle opening gets rid of this, tried the heater fan but no difference. Seems to come from behind the glovebox area.

My Daughters old C3 1.4 did that, the tappets make a slight noise,because it did I changed the oil/filter, it helped but it returned after about 2k miles so left it, it didn`t affect its running so it wasnt worth replacing the tappets.

you can also on occasions hear the injector solenoids, and heard several people complain about that

Bearing in mind I have not heard it so it may be totally different problem?