Plan A is to have both Solar Earth-Charging with solar panels, and Solar Thermal Water Preheating with a storage tank and one solar panel. However, apart from the additional cost of the panels and the extra tank, we would affect the performance of the Heat Pump (favourably), but making it difficult to assess the sole contribution of the Solar Charging. As 75% of the workload is Space Heating, we know that this is the component that most needs solving.
Plan B is to have solar panels linked directly to the ground loop, charging up the deep boreholes throughout the year. Relatively cheap, amazingly simple, but not been done on a domestic property yet! - and we not do Hot water as this would confuse the data collection process.
Plan C requires that we do Plan B first. If it works, we are all happy, and the extra Coefficient of Performance is assessed, and then we could consider whether it is necessary or beneficial to add an extra panel for a Solar Preheating tank. This could be on the west roof, with a Velux added for maintenance access.
Plan D requires us to do Plan B first. If after two years, the experiment was ineffective, and cannot be made better (eg by an extra panel), then we stop off the glycol collector loop and simply instal a water preheating tank in the loft using solar thermal panels on the west roof - which we already know will produce benefits.