Basically the worry over EH's proposal saying extra tunnelling will be necessary, seems to have been misunderstood.
The report actually says EH want to go into the old tunnels, record the open surfaces and backfill the tunnel with material as close to the original as possible. The only way any additional material would be removed (according to Fachtna McAvoy of EH today) is if for instance they have to disturb material when removing the corroding tunnel supports to safely backfill the tunnel.