Why didn't I think to check FreeReg?! Thanks Trish!
Well I don't seem to be getting far...
Is the Thomas who's baptised in 1745 at South Harting, parents Thomas and Mary, the same Thomas who married Ann/e Jackson in 1760? If so, I think it's unlikely he married at 15, so was he baptised later on, as a 5-15 yr old? I couldn't find any other Thomases marrying in Sussex between 1765-1785 on FreeREG.
If this IS the right Thomas, born who knows when, are they even the parents of my ancestor Henry b.1781-4, born 20-24 years after their marriage? And if so, there must be other children before him?
There are a few Sussex baptisms with parents Thomas and Ann/e between 1762-1781 in different parishes:
Thomas - 5 Sep 1762 at St John the Baptist, Westbourne,
Mary - 4 Nov 1764 at St Mary, Stoughton
Fanny - 14 Feb 1779 at St Peter, Racton
Harry - 1 Jul 1781 at St Peter Racton (is this Henry?)
Are they all the same family, and if so, why such a gap between Mary and Fanny? There don't seem to be any other children in other counties (not on FreeREG anyway) and I haven't found deaths for this time period on there either.
Next: is the John who's baptised at same church in 1742/3 to parent Thomas (no mother mentioned), the same John who married Ann Davies in 1775, and the father of William b. 1783?
I feel as though I'm clutching at straws to try to show either sibling OR cousin relationship between the two convicts Henry and William GREENTREE who were tried and transported together. How do I answer any of these questions?

Thanks to all who've helped so far!