janet and john brennan have spent eight years and hundreds of thousands of pounds turning barholm castle , in dumfries and galloway , from a ruin into a stylish castle home .
they bought the 15th century fort , reputed to have been used by leader of the scottish protestant reformation john knox as a hiding place , for just £ 65,000 .
but now it is on the market for offers over £ 695,000 even though at that price the couple will lose a six-figure sum .