If the tensioner is on the way out and you want to replace the pulley why don't you just bite the bullet and replace the whole dam thing? It's a straight forward nuts and bolts job as described by Larry. Cost of the combined assembly is about 150UKP from the dealer but I got one from an importer for half that.
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)