Hello Nigel, I did ask the question (see below). The word 'tunnel' made the Stonehenge WHS Coordinator a bit twitchy but she took in good humour.

I asked a few questions about roads, specifically the A303 and possible tunnel. Before any commitment is made to a tunnel (or its length) there will an ICOMOS Historic Impact Assessment process to evaluate impacts on the ‘Outstanding Universal Value’ of the WHS.

I admit to not knowing much about ICOMOS until yesterday - it looks as though they have made a statement on the subject of the tunnel and its length, see their website.
http://www.icomos-uk.org/icomos-uk-home/

To quote from their web-site:
"ICOMOS-UK appreciates the very real need to address the issue of the A303 at Stonehenge and welcomes the Government’s announcement to do just that. We recognise that a tunnel could help to preserve the Outstanding Universal Value of this important World Heritage Site.
However, we are concerned that associated portals and dual carriageways could have a significant and adverse impact on archaeology across the wider World Heritage landscape and believe that any such threat should be very carefully considered as part of a full impact assessment of the proposed project, taking account of the United Kingdom’s obligations under the 1972 World Heritage Convention."

If Heritage Action want to write an additional piece (I know you already have done so) on the issue of the tunnel, please feel free to use any of the information in my previous two posts.