Iain, Thanks for emailing the Will to me. I'll email a full transcription (insofar as I can decipher it!) to you in a while.
However, I'll just mention that the Will was actually
drawn up in January 180
1. This date suggests that the Aston Rowant records of a Joseph BAGGS who was father of a Robert bap in 1771 (found by DudleyWinchurch) and a Joseph BAGGS who died in 1809 (found by rreadings) are probably one and the same person, and very probably the right chap

It would be interesting to know the age of the Joseph who died in 1809. Perhaps he might have been a brother of James of the Will?
The 1801 Will was proven in 1818 - obviously James hadn't bothered to amend his original Will in the intervening years (and it wouldn't really have been necessary as residual provisions were spelled out in detail at the time the Will was written).
The date when the Will was drawn up is clearly stated as follows:
"Witness my hand this Fifth Day of January One thousand eight hundred & one
James Bagg"
But another clue to the fact that the Will was drawn up considerably earlier than the date (1818) when it was proved is that it has references to the three daughters which indicate that they were under 21.
From the baptismal dates you've given for Ann, Louisa and Harriett, they would have been well over 21 by the date of James's death (1818). They would have been aged 33, 31 and 28 respectively in 1818 but would have been under 21 - 16, 14 and 11 respectively - in 1801.
All the best,
JAP
PS: As far as I can determine, the 1830 entry in the margin doesn't help - it's just passing administration of the Will, following the death of James's wife Ann, to one of the daughters - described as Harriott KNIGHT wife of Thomas KNIGHT.