Comfrey Out of Control

How does one get rid of a patch of comfrey which has overtaken the garden including two elderly but still magnificent and much loved rhododendon bushes?

Comfrey roots penetrate deeply, as much as 1 metre (3 feet), and since each small piece can develop into another plant, comfrey can be difficult to eradicate. The key is persistence. But not of the sweaty, backbreaking kind.
Cut back the plants to ground level. Cover the area with a strong plastic sheeting, at least 4 mil in thickness. Extend the plastic 60 cm (24 inches) past the outer edges of the comfrey patch. Then weigh down the plastic with 10 centimetres (4 inches) of clean soil. In a year you can remove the sheeting and the roots will be dead from lack of light.