You could just replace the bearing. Very easy job. Theres a guy on ebay selling bearing kits for £10 I think.Its better than the cost of a new unit.
Are you sure it's the tensioner that is squeaking mine makes a hell of a squeaking sound and has from new and LR say it's the belts even had them change them under warrentee (those were the days) sometimes when damp sounds like a matalic knock.
go to landranger at deeside then, they are behind b & q neil is very helpful. or cymru is just off the st asaph turning on the a55. dave is also helpful. good luck.