The annoying squeak on my Discovery 1 300 TDI was driving me mad, so I've replaced the ancillary pulley (idler) with a new one – it's part number ERR7295 and quite expensive for a landy part. Stopped the squeak all right and the bearing was seriously nackered. So I've silenced one, but now found another… the tensioner, so back off to ebay for part number ERR4708 or well in my case just the bearing for the part. It's a lot cheaper and it's the only bit that's broken, so it's not worth replacing the whole tensioner unit. So hopefully by the weekend I will be fighting a snap ring to get the old bearing out.
There is a good guide to replacing the bearing on the Landy World web site.