If family history is accurate he didn't die in the tunnel itself since the bends doesn't present until after the atmospheric pressure has changed. I didn't know where he might have gone before symptoms presented, but did hear that he was in agony wh3en he went and that it was a blessing that his suffering ended.

Well the Severn Tunnel flooded several times during its construction and divers were used to assess / start the repairs - so its possible that a number of decompression sickness / illnesses may have presented in the construction teams.