I have a horse chestnut tree which is suffering from the above fungus as diagnosed by the Local Authority Tree Officer (he believes this is it but cannot be sure without tests). The Tree Officer thinks that as the fungus is developing quickly it may have made the tree unsafe so that either remedial action needs to be taken quickly or the tree taken down. Please advise as to what remedial action can be taken and how easy is it to establish the extent of the wood decay caused by the fungus. Clearly I would like to avoid taking down the tree but if it is unsafe then I would have no choice.

Horse chestnuts do not resist decay well.
Someone with a PICUS would be able to give you a good idea of the extent of the decay but there is no guarantee.
The tree could be crown reduced.
The likelihood of death,injury or damage needs to be taken into account when assessing the tree for removal.
One large bracket may be ok, several small ones all around the stem is bad. Sounds like you're on a timescale for destruction with the the tree, the question would be; how big's your bank account??