Thanks, Heather, for all that extra information. I expect you've got the dates of the Fawley baptisms, but in case not:
* Thomas, illegitimate son of Thomas MASEL [sic] and Mary FLIGHT, on 6 Jan 1771.
* Betty, illegitimate daughter of Thomas MASEL [sic] and Mary WILTON [sic], on 28 Jan 1771.
* Mary, daughter of Thomas and Mary RASEL of Ower, privately on 17 Mar 1776, received on 7 Jun 1776.
* Ann, daughter of Thomas and Mary RASEL of Ower, privately on 2 Nov 1777, received on 1 Feb 1778.
* Sarah, daughter of Thomas and Mary RASSEL of Ower, privately on 9 Jan 1780, received on 25 Jan 1780.
* Hannah, daughter of Thomas and Mary RASSELWALTHMAN [sic] of Ower, received on 4 Nov 1781.
Thomas, Betty and Mary you know about. I guess Ann, Sarah and Hannah died before Thomas (the father) wrote his will. I can't see burials in Fawley, but maybe they died elsewhere. I can't help with a baptism for Harriet. However I do have one burial that I find hard to explain. A Mary RASELL of Fawley was buried on 22 Jun 1777 in Beaulieu. If this wasn't Thomas and Mary's daughter, who was it? It can't have been the mother as two or three more children followed. Do you have any ideas? I wonder if it's possible that their daughter Mary did die in 1777 and that they had a later daughter of the same name. Do you have an age for Thomas TULK's wife Mary that might help us determine this? The marriage was in 1808 which would easily allow her to me born later than 1777. However there's a Thomas and Mary TULK living on Britain St, Portsea on the 1841 census, both given as aged 60-65 which is consistent with a 1777 baptism, if it's them.
To return to Mary FLIGHT, do you have any indication of her age? There are two possible baptisms in Fawley, and none in the nearby parishes:
* 10 Jun 1744, daughter of William junior and Mary FLIGHT; received in church on 3 Jul 1744.
* 10 Feb 1749/50, daughter of "Farmer" FLIGHT; had twin sister Martha.
There's a burial at Fawley of a Mary FLIGHT on 25 Feb 1768 which could be one of these children, but more likely is the mother of the first. I'm not sure who "Farmer" FLIGHT is. It could be William junior, in which case the first Mary presumably died; or it could have been William senior, who I suspect was William junior's father and who had recently taken a much younger second wife, Sarah Wright; or it could have been someone else entirely. But at the moment my guess is that "Farmer" FLIGHT is the father of William junior, and therefore the younger of the two girls named Mary baptised in the 1740s was the aunt of the older one.
There are two marriages for a Mary FLIGHT. One is at Lymington to George WARE on 13 Nov 1783; the other is at Fawley to James BANKS on 17 Aug 1784. I cannot say which marriage goes with which Mary FLIGHT baptism; nor do I know which of these women was the mother of your ancestor, Thomas FLIGHT / RASSELL, but presumably one of them was. George and Mary WARE had two children, George (born 8 Sept 1787) and Mary (born 10 Aug 1790), but either of the children baptised in Fawley would potentially still have been of child-bearing age by then, so we're none the wiser. So far as I'm aware James and Mary BANKS did not have children, and I suspect James died a few months after the marriage, so that doesn't help untangle which Mary FLIGHT is which either.