...and now I think I've traced this CORBETT family through to an 1802 birth of a John Corbett, the grandfather of the John CORBETT who married Nellie Harper, so all Huntingdonshire through and through. Sorry I troubled you on this Irish board, but I think the family myth is well and truly done away with now.
In my searching I did come across a "Killed In Action" at Gallipoli on 18-08-1915 for an individual called John CORBETT from St Ives in Huntingdonshire. This could be the same man mentioned on here, b.1877, and might explain why there were no more children to him and Nellie after Olive in 1914.
Can't find his name on the War Memorials for St Ives or Houghton and Wyton, so I may be barking up another wrong tree...
Anyway, thanks everyone who has helped me on here.
Keith
Ah, found him, on the Eynesbury War Memorial, his last known 1911 Census address being St Mary's Street Eynesbury...