As has been said a few times here, an accumulator tank as large as possible (min 700l @ 50l/kw from the BBU), use that to heat an unvented cyl and heating zones. Put the solar output through an immersun or similar to divert any extra energy into the unvented cyl.
Don't forget the Back End Protection / Load Unit on the solid fuel boiler... Not only for protection but also to make sure you are getting only high grade heat pushed to the accumulator. Nothing worse than pushing warm water to the top of a tank and effectively destratifying the thing.
Remember your safety controls, overheat stats etc to make sure that everything is as safe and efficient as possible.