Another one from long ago... Trident landing at Heathrow in gin clear conditions. The IRVR suddenly came on with a very low reading so I read it out as I had to. The Trident crew asked if there was a problem with the IRVR and when I said there was not they said that they were bound to go-around and promptly did so. We heard that the IRVR system has been turned on for maintenance but nobody told ATC!