Hi Linda
I'm very grateful for your help, and it means that John's branch of my family tree is now taking shape nicely. Yes, the information so far suggests a date of birth for John of c1825-c1827. I've searched through the historical records on FamilySearch, and found a John Knill, son of Matthew and Elizabeth Knill, baptised 14 October, 1827, in Swimbridge, Devon, and a Matthew Knill, son of Gilbert and Elizabeth Knill, baptised 23 February, 1760, in Chittlehampton, which may be relevant.
With thanks
DevonKnill