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Sooty bark disease? Lop the sickly ones out or leave? Whatchya reckon?
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It's an invasive weed, get rid of it and plant a good native species.
Exactly what cheezybeanz said. Sycamore is rubbish. Plant oak or Ash.
Or a nice English Wallnut.
Well, yeah but it serves a purpose at the minute, sheltering our house and garden, there are maybe 20 sycamore with ash, elder, hazel, alder and blackthorn interspersed ( including the daddy sycamore which is huge and we'll over a hundred years old). Am working on thinning the sycamore a bit at a time and getting light to the others but this runs along a beck so not the easiest to plant additional trees, walnut's a nonstarter. If I leave the diseased sycamore is it likely to infect the other trees before autumn when I can yhin them out a bit more (loads of birds in them at the mo)
If I remember correctly, sooty bark is endemic and the tree will succumb if otherwise stressed. Coppice it and hope for some regeneration.
Neighbour across the road has a mature sycamore in his front garden. It really is a filthy tree, all his gutters are choked with whirlygigs.
Sooty bark will kill the sycamores and might get any Norway or field maples. All the others you mention are immune and ash will usually seed themselves well anywhere that sycamore grow well, so they are likely to start taking over. It might take time and unfortunately there is an ash die back disease, but natural regeneration is cheaper and often gets results quicker than planting. Also losing the sycamores will give more light to other trees, which might do better.
In defence of sycamores they aren't native, but have been naturalised here for a long time. Not the best thing close to houses but provide greenery in places not much else will grow - for instance the gap in Hadrian's wall seen in the Robin Hood film a few years ago. They get bad press on ecological grounds because they have a wide range of associated insect species (about 15, compared with 280+ on oaks), but the large biomass of aphids on a typical one can support a wide range of birds