Thanks again Both,
Raybistre, I apologise for my error in the earlier comment. I though that I had read that mother's maiden name only started appearing on GRO records in about 1911 (I think). Obviously I was wrong as I have now searched for Emma Jane (Thomas and Elizabeth's daughter, my great grandmother) born in 1878 and the mother's maiden name does appear. It shows as Poole.
That does underline the reliability, or otherwise, of spellings. I now have Pool and Poole and Davies and Davis. I am fairly certain that it is Davis and Pool.
So, another journey begins although I think I may need some luck searching for two Thomas Davis in Montgomeryshire.
Thanks again.