Patsy, yes, our neighbour has a fine bunch of the stuff growing on his small rowan tree. The tree has a sickly branch, but NOT the one with the mistletoe on it! I would imagine it sucks goodness from the tree, as it has no roots of its own. But a parasite usually doesn't kill its host.
Anybody else, apart from me, puke at Cliff Richard's Mistletoe and Wine song? (Sorry Cliff!)