thanks pache and villa.
plumber now steering me away from ufh. have read so many posts on subject that i'm also thinking money better spent upping spec on insulation if that's still poss. at this stage. will stick with solar for the DHW and compliance with building reg.s. should i do this, i have been advised that i will only need 1 stove +BB. am thinking of this fine wire HRV thing for ventilation. the stove would be running fairly regularly, though not all the time. head bursting with worry as hole in ground nearly full, decision time approaching fast and engineer to keep happy!