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Following up my own post, I have now found the circuit diagram (http://wiki.openenergymonitor.org/images/EmonP_V1.5_sch_white.png) and it looks like a relatively simple small circuit of a bunch of resistors, one cap and a diode. Has anyone already built this externally?

I feel that I am just being cheap in avoiding the £60 for a emonTX. Which given my lack of time and the many years since I built anything from components, I may just have to do to keep it safe and simple.

Ah hell, I ordered an emonTX to make my life easier. But if anyone wants to fill in the background for future readers...

I don't see a reason why you cannot add the signal conditioning circuit (the parts you describe as "a relatively simple small circuit of a bunch of resistors, one cap and a diode") externally, feeding into ADC6 as you suggested. But only 5 V is available on pin 2, not the 3.3 V that you require to use the same component values, so you would need to recalculate the value of the 'top' resistor (R27 in the CT1 circuit) so that the midpoint was biased to 1.65 V and not the midpoint of 5 V. 910 kΩ would be suitable, alternatively you could keep (R27) at 470 kΩ and reduce (R25) to 240 kΩ. (Both pairs of values don't quite bias the midpoint at exactly 1.65 V, but the difference should not matter too much - it will just reduce the maximum current that you can read by a tiny amount, about 2%). But it might be a good idea to add a 100 nF ceramic in parallel with the 10 μF, because when I put my meter on the 5 V rail, it was waving about as the LEDs came on and off, so there's digital noise on it.

Thanks, like I said to make my life easier I ordered a emonTX ... but maybe someone else will find the info useful!