"Any comments/ suggestions very gratefully received!"
Remember you asked for it.
I have been thinking about this a lot lately and would go with #1. No question.
The pumps don't use much power, maybe 10 or 20W, and they don't run 24/7.
Stray thoughts...
Those days when the temperature isn't too cold, the floor will be warm. Might not need a fire that day.
Could use 2 smaller pumps and 2 thermostats. One pair set at say 70F/21C, the other set at say 68F/20C. I believe it would be more efficient, but if 1 pump couldn't keep up on cold days, the other would come on. Plus if one quit, there is still one that works.
Could run an extra seperate line in the floor for solar. Fairly cheap. Not sure if that was the plan.
Here, when it starts getting cool, we still have quite a few sunny days left to come. Could certainly postpone the heating season a while.
Plus it could give a jump start on heating the ground. I wondered about solar heat pipes a considerable distance below the regular pipes, then start the solar heating season a week or 2 or 3 early. Heat all that 55F/12C dirt up a bit when the free BTUs are available.
Could thermosyphon, at least some, in a 3rd seperate pipe. I know it would work.
Some local people heat domestic hot water with a small wood/coal/kero heater in summer.
In winter they use the home heating system for domestic hot water, however it makes for some water temperature issues. They manually partially open a valve if the water temperature gets a little to high. The colder it is outside, the more they open the valve, because the more heat the stove is making. The valve feeds a thermosyphoning loop with about 8 to 16' of copper pipe with press fitted aluminum washers (like the guts of a radiator, and usually placed under the bathroom floor) which removes enough heat from the system to keep the safety valve shut.
The water returning to the heater is still VERY hot.
Open a valve to a small radiator considerably higher than the heater, then drop it down into the floor pipe, then back to the heater. The beauty of that is it runs 24/7 without power. It would not usually keep up and manual control is probably best, but whatever flow it makes keeps the pumps from running the same amount. I believe it could be considerable.
Another benefit would be moving heat to the areas of the house far from the wood stove. Run the thermosyphoning pipe to the other end of the house before making loops (maybe even lightly insulate the run to the loops?). The areas near the stove already get heat from the stove.
Could, I think, use all the primary pipes with 2 staged pumps. It would lower the head due to friction (less battery amps) with only one pump running, at almost no additional cost.
I'm not sure, but expect most pumps have considerable resistance to reverse flow. If not, use a check valve. Maybe a horizontal check valve placed verticially would work with a lot less drag.
Be aware the controls for DC are not readily available (in the US). They are not hard, complicated, or expensive to make, but I couldn't buy them off the shelf. DC is far better than an inverter, so start looking at design parameters now.
What I recently did, though if it was for personal use I would have made something 100% solid-state, thermistor controlled, and 12V.
I didn't mention the 2nd thermostat measured if the stove was burning or not, because if the stove was NOT burning it prevented the pumps from running (wasting battery power if no heat was available).
http://www.fieldlines.com/story/2007/11/5/15930/5940
This is a coal fired water heater using thermosyphoning to a standard 40 gallon HW heater used only as a holding tank (behind the wall). Takes about 10~15 minutes from dead cold to plenty too hot for a shower.
It would do a much more efficent job heating the house in cool temperatures than the giant gravity-air thing (almost shown) on the right. They must often open windows while 32F/0C outside.
http://www.otherpower.com/images/scimages/2050/watrheatr.jpg
I'd suggest running a few 'extra' pipes in the floor too. Cheap to do. Adds options later.
G-