TOM HART DYKE has a bit of a thing about plants. You might call it an obsession. You might call him certifiable, in fact. But it's a truth universally acknowledged that a single man in possession of a large ramshackle country estate and an obsession with plant collecting could want for only one thing - in Tom's case it's a walled garden at his family home of Lullingstone Castle in Kent, containing examples of plants collected from every corner of the globe.