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Thanks for your answer and i agree that i should consult a heating specialist before spending that kind of money. I think a few extra pound would be money well spent to get the best professional advice instead of taking a chance and finding out that the heat output is not sufficent.
The boiler wouldn't be replacing anything as its already there. the heating system is dual, which means we can use solid fuel in the range or oil with the boiler, to heat the water. The boiler would remain but i would hope to replace the range with a stove as we never use the oven in the range.
By the way, The house is a 4 bedroom, bathroom, kitcken, utility room and living room bungalow thats poorly insulated because of the age of the house.