It sounds like you will want to check out ENVI's mosaicking tool, which you can find under Map > Mosaicking. Of course, a mosaic of several Landsat images will be a pretty big image. So, you may find it useful to subset your different Landsat tiles first, to include only the areas of interest to you. - Peg

Thanks a lot for your solution and advice - exactly what I need and much appreciated. I don't suppose you know how to overlay images so that where there are clouds in one image I use a second image which is cloud free in that area to replace the clouds in the first?