I think the phrasing is along the lines that United Utilities require the tunnel to be investigated by engineers, hence the Grosvenor have been asked not to use the tunnel.
Presumably the inspectors will be far more familiar with the condition of Milwr than people who go in there regularly - I'm sure that GCC would be far better placed to advise whether Milwr was deteriorating, but they probably don't have the requisite dimplomas in bafflement.
Hello again darkness, my old friend...