One thing VBB, the air and moisure getting tro some parts of the Neolithic surface is down to the failed tunnels and if these were simply fixed the archaeology would be sealed again. This fact means that simply expanding the existing tunnels as part of a repair stratefgy is unlikely to find any decent evidence - causing the archaeologists to want an even larger hollow to be made - a dangerous thing in my opinion.
It should also be said that EH don't seem to think the archaeology is in much danger and I would suggest that only a proportion will have been exposed to air and water, and this exposure is directly down to EH's poor management of the hill for the last four years - so how can they we trusted to get what will be Silbury's biggest intervention ever right?