Thanks for this, Jan - definitely a route to investigate. Interestingly, Jonas' name for his first four children was recorded as Jonas in parish records, but for his last two (both John) it was recorded as Jonah. Spelling was indeed cavalier, to say the least! The only downside of this is that, with naming traditions in those days, Johans' parents' names, Thomas and Martha, could reasonably be expected to figure in younger generations, but there is no mention of them in the Rich family.
As you say, it is something to check out more closely for that parish. But thanks again for your help.
Amlodhi