There is a John F. KOWLANS aged 2 born and living in Hartlepool in 1851. He is indexed as KOWLANS, his parents Patrick born 1816 Ireland and Jane and other children are indexed as KNOWLAND
HO107/2384/505
This must be the one Barbara found. Unfortunately the wrong family. Suppose your NOWLANS could have been from Ireland and maybe returned there?!?
Thanks so much for your replies - I've only just got back from holiday, hence the long delay in replying.
This one is a real puzzler, I have looked everywhere I can think of up to now, and, like you, am coming to the conclusion that they might have emigrated.