Revisiting this, we may now be going down a completely different path (which I feel little, if any, regret about). We're looking at a third party solution which would do exactly what I want it to do. The only problem is the price, and the fact that only the Enterprise license provides the required API. On the other hand I think I can justify the price tag as to build anything more than simple, static reports ourselves, let alone coming close to what this one provides, would take six months at a bare minimum and probably longer.

It's also worth noting, for those who are looking to do some more detailed SharePoint usage reporting on SP2007, that they also have a Lite version available for free.

The product's CardioLog and I'd be interested to know if anyone has prior experience of this or any other information which might be useful. I'm currently trying to schedule a phone call with them to discuss requirements and will be able to update after that.