On idle, engine very very smooth, but you can faintly hear a cricket type noise.

 

Is this idler or water pump...or something else?

my alternator or the belt makes this noise when the battery is low, see if the noise gets louder when you put a load on the electrics (while opening window or turning headlights on etc)

Nope, never gets louder as it's quite faint. Sounds like the noise is coming from quite low down. Can't even hear it with the bonnet open, but when shut, it sounds like it's coming from somewhere near the bottom of the rad....that kinda area.

you hear it while stationary? if you hear it when driving over a bumpy road then its the bonnet catch rubbing, stick some electrical tape round it and no more noise.

Nope, I've solved the squeeky bonnet syndrome already. This is something else.