Auto dimming Mirror

Hi, anyone know anything about these? I`ve got one on my 300tdi ES but it can`t be working...are they supposed do go darker across the shiny bit or something?? Would a `tronic guy be able to fix it??
Anyone know where to get a replacement apart from landrover which will charge ££££`s??...tried the scrapyards in the mags, none about( of course we would never swap one over during a test drive would we???)

Mine only dims when there is someone behind with high beams on at night, quite subtle but noticable. Its great for when the dirty big road trains approch you with their bull-lights on that soften the asphalt if they drive too slow!!!

It gives the same result as one of those toggle switches on the mirrors but without any input from me.