My wife is the great-grand-daughter of George Bird born 1815, are you related?
I think my information mainly confirms your own but it goes like this,
There were three Giles Rendals in baptised West Coker namely,
[1] Giles son of William & Frances Rendal bap. 12 July 1789, he may be the one buried 23 April 1831 aged 42 - known siblings John bap 29 Nov 1798
[2] Giles son of Lot & Mary Rendal bap. 5 Jan 1790 - known siblings Ann bap 16 May 1785 and William bap 20 May 1798
[3] Giles son of Abraham & Ann Rendall bap. 7 July 1793. Known siblings include John bap 25 Dec 1785, Mary bap 8 July 1787, Jane 12 July 1789, and Samuel bap 12 July 1795. There must be others since they buried at least three more, Elias bur 25 March 1792, another Elias bur 17 Dec 1797, and Edward bur 21 Jan 1798. Probably all these families were baptising somewhere else as well but I haven't found them.
There also seems to be a fourth Giles, buried aged 59 21 Sept 1844 who doesn't fit any of the above.
Giles Rendall married Mary Rendall 29 Nov 1813 and produced Ann bap 18 Sept 1814 she eventually married George Bird 11 Oct 1846, although we believe she then had an illegitimate daughter Ann aged about 10. It seems to be a bit of a habit in West Coker. My bet is that Mary is the sister of Giles [3] who was a witness to their wedding. But I can't make out whether the husband is Giles [1] or Giles [2].
If that is right then Mary's parents are Abraham Randal mason and Ann Penny, married 8 August 1785. I haven't so far traced either of them further back.
How does this fit your data?