"she told me it was being looked at again nearer the time (not 1968) to evaluate if it would do more harm than good."

Well VBB, let me run this past you and the nice lady. Its close to the surface isn't it? So that means its not bearing a load, in fact its nor doing anything except forming part of an opening into which a door has been fitted. So when the jobs done, and the front of the tunnnel is packed tight with chalk, if the lintel is lifted up it will have no structural effect whatsoever and will simply leave the oblong hole that was dug in 1968 to accommodate it. The plan is to fill the slumped area up-slope from it with fresh chalk to reinstate the pre-1968 profile of the hill so adding a couple of bags of chalk into the lintel hole at the same time would seem sensible.

The whole process would be the exact reverse of the sequence in 1968, so it would be an actual and symbolic reversal of Atkinson's intervention, which is precisely what EH say their whole project is about. So it seems an obvious thing to do. In addition, I'm unable to think of how it could do physical harm. Could you ask the nice lady if she knows otherwise, and if so what the harm might be? Personally, I'd like to think there was no structures left that would make it easy to fit another door some time in the future, but that's just me I suppose.

I could Ask the Experts I suppose, but I'd much rather have an answer.