Actually, I used the Cannondale and it was brilliant.
I need to sort the phantom noise, and the seat has to go, but a really enjoyable ride. Encouragingly, the bits when I pushed it a bit were the most enjoyable. I got a bit of cramp above my left knee towards the end, and above my right knee shortly after (as I used my right leg more to compensate) but that was largely due to the heat and shifting around constantly in the saddle.
So I'm going to stick with it for now! It's going in to have the noise investigated, then I might invest in a proper bike fit. New seat first, though! How can a Charge Spoon be so comfy, and a Fabric Spoon be so hideous?