Sadly, this is a case of unrequited love. I do not want to leave any surplus cash with EON, especially when they haven't mentioned paying me any interest on the credit - or even putting the interest toward my bill.
But on the upside, EON's letter did prompt me to call @Oikonomics, who is fairly heavily involved in an alternative energy proposition, amongst many other things. We got talking about Green Deal and feed-in-tariffs and the challenges of comparing the official option for financing your personal alternative energy generating/saving with alternative means of financing. One idea is to enable you to generate an "Energy Credit Rating" to use as a base for comparing the overall benefit to an alternative energy option. It could combine your MPAN billing number, linked to some data from the Met Office about average sunlight/wind in your area, local energy prices for what you buy and what you generate, the Feed-in-Tariff scheme, and your own credit rating.
Linking some of this data may also help reduce the complexity/cost associated with making energy price changes.
Oh, and Bruce also told me Ovo credits your bill with the equivalent of 3% interest on any credit balance (see under "Earn from Energy").
Image from Energy Saving Trust.