Thanks for your response: interesting and enlightening. If only other posts on this thread (including mine, it has to be said) were so considered, I wouldn't be so grumpy today! I agree with your response, viz:
"I would say the nearest you can get to 1958 would be to leave it alone or if anything grout - but certainly not rebore the tunnel."
If I knew how to put the last bit in italics I would! :-)