Morning Pippa,
Which Archives did you apply to for information? That page number definitely exists at The National Archives. It should confim which Battalion Ernest was in. This will in turn make or break his story. I have to say that I have seen many examples of the story told does not quite match historical fact. In many cases, such as Ernest's, I think it was changed to help their mind cope with what they saw and did. There are Prisoner of War records held by the International Red Cross in Gerneva. For an example of what you can get put "Connor" in the search box at the top of the page. Up will come a bit of reading but you will see a typical reply. It will show when and where he was captured and all the camps he was in. The bad news is that they make a charge for this information (I have edited this comment as I have just found out that for next of kin no charge applies). Do you have access to Ernest's medals? I would to know which regiment is stamped around the outer edge please.
Regards Mike