Right, just starting to get back to this now after being pulled away by family stuff. Have picked up a Tsunami (basically an Arduino coupled with a basic DDS waveform generator and frequency/phase/amplitude detector). It is almost certainly too slow to pick up a decent glucose signal (or possibly anything) but it will let me get some of the basic ideas straight before I start designing PCBs, wrestling with high frequency signals, buying possibly pricey DDS chips and so on. Regardless I'll keep posting here.
I've had one or two offers of help/interest and I will be following these up too.