A letter from Network rail's John Halsall (click for a larger version):
"...we do not consider there to be a material risk of failure of part of the tunnel lining without prior warning in the form of significant cracking of the tunnel wall."
How quickly would cracking of the tunnel wall be noticed? How often is the tunnel thoroughly inspected? It needs to be inspected daily until the exact location of the 1849 mine gallery is determined and the mine entered and surveyed.
"The drilling should establish if there is any evidence of the mine/cave referred to in the 1849 article beneath the tunnel wall or track at the location we agreed and whether or not it was filled adequately at the time."
I think "should" is rather optimistic, bearing in mind the limited depth and scope of the proposed investigation. This statement again confines itself to the 1849 article and ignores the possible existence of related unknown mine galleries elsewhere under the tunnel/s. Furthermore who is to judge what constitutes "filled adequately"? We suggest an independent engineer selected by ourselves and paid for by Network Rail.
Per von Scheibner has mentioned that the letter omits parts of their agreement, i.e. the depth (2 metres) & positioning of the boreholes (not just under the actual rails, but also between the rails & the tunnel walls on both sides of the rails at one metre intervals along the tunnel entrances on both sides).
Network Rail are - as I expected they would - doing the minimum they possibly can to investigate this issue. Furthermore the transparency of any investigations carried out by them (past, present or future) is open to question. Unfortunately I see a return to using the legislative brute force of Parliament and the House of Lords in order to make the obstinate elephant that is Network Rail move more than a centimetre towards fulfilling its duty of care to the travelling public.
Go on Network rail - surprise us: do some proper (and transparent) investigations, and do them now. Not in 6 months or a year. NOW.