Having picked the brains of SusieGee's husband, we have a system very similar to theirs, i.e. solar panels and wood burning stove both heating a thermal store, which provides heat for both underfloor heating and hot water
There's one point where I went against conventional wisdom, though, and that's on "bigger is better." The bigger the store, the more energy you can store in it, but the more energy it will take to get it up to temperature. For some systems, e.g. underfloor heating, it may not matter if the water is only lukewarm, but for others, e.g. hot water to the taps, you'll probably want it hotter. Stores are usually designed to 'stratify' so that you get hotter water at the top and cooler water lower down, but I've heard that this can be a bit temperamental, so I didn't want to rely on it.
I calculated the size of our store based on what we need to get out of it: Heat the house for a couple of hours in the morning, plus one shower. After that, we can light the fire to heat it up again. Our store is 350 litres and, though the system is only half finished (we don't have the solar panels installed yet, which reduces the effective size of the store because there's a solar loop full of air. On the other hand, the shower is not yet connected to the hot water system and still runs on electricity), it works pretty much as designed.
The main reason, as far as I can tell, for having the biggest store possible, is to absorb all the heat being generated without overheating. This isn't really an issue for us but with solar panels in Greece, you could easily get too much heat. It is essential that you have somewhere for the heat to escape to. We have a radiator in the bathroom - it's supposed to be on all the time, but our plumber was kind enough to fit valves so it doesn't shed heat constantly.
There are decisions to be made with this type of system about whether each heat exchange (water being heated up or giving heat off) is direct or indirect. We have four exchanges: Stove and solar providing heat, and underfloor heating and hot water taking it out. Our stove heats the water in the store directly, which means that it's the same water in the store as in the boiler on the back of the stove. The solar panels will heat indirectly, meaning there's a coil in the store through which water from the panels will flow. The underfloor heating is heated directly, i.e. same water in the store as in the underfloor pipes (except that there's a clever bit to send the water round and round the pipes if it's too hot, until in cools down then more hot is added in) and the hot water to the taps is heated indirectly, by sending mains water through a coil in the store. This means that we get mains pressure hot water and we don't have to worry about having lukewarm water sitting around breeding Legionnaires disease then coming out of the taps.
A note on thermosyphoning systems: Even if this wasn't a requirement (and I'm not sure it is in the UK - one plumber certainly tried to persuade us to have a pumped system), I wouldn't be happy relying on a pump to take heat from the stove to the store. If you have a power cut, you have to put the fire out! We have a thermosyphoned system with the store on the same level as the stove. I was surprised that this would work, but if you have a roughly circular pipe run, the hot water will rise up out of the back of the stove creating a current round the loop, with cooler water sinking out of the bottom of the store and returning along the lower pipe to the stove. It does work. The only downside is that it also goes backwards. When there's hot water in the store and the stove is cold, the hot water heads from store to stove. Regulations apparently prohibit any kind of obstruction in this system at all (in which case I'm not entirely sure how pumped systems are allowed. Very confusing) so we can't have a valve to stop the water flowing backwards. You might want to consider this sort of arrangement especially if you go for a very large store. The larger the store, the stronger the floor supporting it needs to be, so you might be happier with it on the ground floor.