Wow! What a lot of replies to wake up to! Thanks so much, everyone.
Liz, Barcombe was where Jonathan was born but his father George could have been from any of the nearby places you and others have mentioned, so I'm too worried about him not coming from Barcombe.
I must admit to still being worried that the marriage to Martha ( and not Mary) can't be right. The 1777 Kent marriage to Mary Skinner might be early but I'm also thinking if I've got the right death for George in 1834 aged 78 (i.e. born c 1756) he'd have been old enough to marry in 1777 and, if the couple returned to Sussex to live, any earlier children's baptisms won't be online either. (as in, no George, no Mary - at this time they usually named some of their early children after themselves).
It may just be that Martha is a complete red herring!! She can't be a second wife because Jonathan's and Mercy's baptisms in 1803 and 1807 respectively, name mother as Mary.
Liz, have you got very far back with the Fletching Chatfields in your tree? Anything going back earlier in the 1700s and, specifically, any connection to Cuckfield, Ardingly or to the surnames Beard or Symonds (numerous spellings)?
I don't know the area very well at all - infact, only recognise some of the placenames from tootling through on the train between London and Eastbourne!!
John, thanks for the links, though I couldn't find many 18th century PRs on Family Search, though I did look at the George/Martha marriage original. I liked the Overseers accounts - giving it the human touch.
There are 3 public trees on Ancestry with Jonathan on and none of them seem to agree. Infact they all but contradict each other, but I can see why now!!
Thanks, Jill