"want a tunnel (say to further the existing limited knowledge about Silbury)?"
There are many research options available that would not need to have a full scale tunnel.
Also a research project should be an University Archaeological option at a moment that this is in co-operation with a larger and wider scale archaeology project with a objective to shed some light on this exact dated timeframe in the pre-history.
I'm not all that knowledgable on the hill, but is there a burial in the hart of the mound? Is it important to get to the hart, tunneling? Would it not be better to first understand the immediatly surrounding area, say inside a two mile radius.
It may be hard to track back all the MD artifacts but with good samples of the earth, and the museums together with some new archaeology 'fine' research experts a understanding will develope as the living surroundings of someone holds far more information than the last resting place.
Once 10 years onward, when this research would be completed I am sure that there will be new and less disruptive ways to research inside the hill.
The importance of the settlements are also an issue at Thornboroughs Lady bridge. The quarry is well aware that 'fine' finds will give us knowledge on the using of the ritual landscape, as are the tenders of the hill.
It is illegal to disrupt places where important archaeological conclusions have been made or can be made. Both are therefore in the wrong and denial is incompetence.