As you've got the Onkyo receiver, I'd suggest Onkyo's DS-A2 dock (£65) that will plug directly into a spare set of phono ins on the receiver, and gives you full charging and remote control of the iPod.
If you can spend a bit more, the Arcam rDock (£130) sounds even better, although it doesn't have a remote.
I'm sure you can even spend more for a more technical lead! They just plug into the top where your headphones go and into the back of the receiver! Just chrage your pod normally as you have been doing.