Do they own enough in agricultural property to be affected? You can still give away £1,500,000 per person, free of Inheritance Tax, is leaving your main residence to direct descendants (£1,350,000 per person if not).
They are in their seventies. I doubt they really care at this juncture. Looking at everything from a whole.
I suppose Jeremy Bamber still fighting to overturn his convictions is a stressful affair to them from which ever side you look at things.