Hi Rosie
I've searched quite a few parish registers from the Cheltenham area, and the best match I can find for you is from Charlton Kings (rather poshly entered in the register as Charlton Regis!), just outside Cheltenham. It's the baptism that ChasH found for you, and I'm going to spoil his fun now (sorry, Chas

) and tell you that it was for the baptism of Henry Turner, son of William Turner, bricklayer, and his wife Mary, date 24 April 1825. Henry's brother Robert was baptised in the same church, St Mary's, on 8th June 1828.
Henry Turner in the overseer's records I mentioned earlier isn't the same chap. He's 39, the son of Thomas & Grace Turner, either in the North Nibley workhouse or applying for parish relief (it doesn't make it clear) with his second wife Mary and the children from his first marriage to Elizabeth, Ann 10, Hannah 9 and Charlotte 4. They come from Kings Stanley, I don't think they can be related to your Turners.
Of course, finding that entry doesn't guarantee that it's your Henry, though it's tempting, as his father was called William, isn't it?
Time, as always, ran out before I was able to check on William and Mary's marriage. There is a marriage on the IGI of a William in Charlton Kings 18 Feb 1821, but it's to Sarah Constable. There are also two marriages for Williams in Gloucester, about 8 miles from C K, the first to Mary Bennett 19 Oct 1815, the second to Mary Leott 25 Aug 1819, both in St Mary De Lode church.
Regards
Gillg