I cannot find any information on this anywhere. Sorry.

It might be a good idea to find out what the disease is, as there is one of the recent tree killing diseases that is affecting horse chestnuts. If it's that you might need to remove the tree so perhaps you might wish to get some professional advice from a tree specialist. Or try putting a photo of the affected part on this site and see if anyone recognises it.

If you are worried could you just collect the others for now and perhaps keep the heap with most chestnut ones separately until you find out?

Its probably horse chestnut leaf miner, the little pests crawl inside the leaves, not an actual disease ( check forestry commission site)
They may advise on composting.

Avoid the sycamore leaves (look up allelopathy), but the others will do fine to keep as the caterpillars causing the damage to the chestnut leaves will have pupated. Ash, makes good leaf mould.

Our leaves are a mixture of ash and sycamore. They have improved the soil texture a lot (it needed it!) and hasn't affected the growth of the mature plants or bulbs. If it actually inhibits germination do you think it would be useful as a weed preventer Hortum? Come to think of it there weren't a lot of weeds in the beds where I used it (except foxgloves and campion, and nothing deters them!)