Hi David, I have actually made some progress since I posted my original message. Ann Mitchell was John Gasson's third wife: he married Elizabeth Todman in 1744, & Elizabeth Head in 1769 - at first I thought that I had simply misunderstood the meaning of his will, but eventually pieced together the fact that he was referring to two different Elizabeths (the first of which he apparently did not like, as he left the children of that marriage only one shilling each, whereas those he had with Elizabeth Head got several pounds each!).
I suppose that the Ann Mitchell you found 'could' be the correct one: since she had her last child in 1791 (I found another one, Jinty, also baptized at Charlwood, on the Surrey baptism CD) she certainly couldn't have been born any earlier than around 1740/45, but 1759 would fit better with the age at which a woman might first get married; however, this would mean a massive age difference between she & her husband - at least 30 years bearing in mind that John first married in 1744, & perhaps an even greater difference if the one possiblity I've found for him is correct (John Garston, b. 1715 Charlwood).
I would feel a lot happier about 'your' Ann Mitchell being the correct one if only her father's name had been Edmond, but bearing in mind John's probable age, perhaps this Edmond was Ann's grandfather rather than her father? I've just been looking at the Worth baptisms on the Sussex fam history grp data archive, where I found Ann Mitchell (baseborn) to mother Ann Mitchell in 1780; so perhaps our Ann was a bit wayward, & her grandfather persuaded his old friend John Gasson to marry her before she brought any more shame on the family?!!
Incidentally, you may be interested to know that I had a fair bit of luck in tracing the fortunes of some others of John & Ann's children: Samuel farmed 119 acres at Lyons Farm, Isflield, & Edward 200 acres at Rowley Farm, Charlwood - the farmhouse is still there, & seems to be sandwiched between an industrial estate & one of the runways at Gatwick Airport!
Anyway, many thanx for going to the trouble of looking that up for me; if you get any further inspiration I'd be glad to hear of it.