If you look at CheshireBMD there is a death in 1851 registered at Winsford of a Sarah Sefton. No age is given for this one although all the other possibilities are eliminated by age. If you send for this certificate specifying the approximate age you should only be charged if it's the right one for you. The GRO reference is Northwich 19 461 December Quarter 1851. The CheshireBMD site has ordering instructions if you want to go through them.

If you haven't found it this looks like it might be her baptism:

Baptisms: 9 Nov 1823 St Elphin, Warrington, Lancashire, England
Sarah Sephton - Daur of Thomas Sephton & Ellen
Abode: Tanners Lane
Occupation: Glass Maker
Baptised by: J Topping Curate
Register: Baptisms 1822 - 1826, Page 64, Entry 512
Source: LDS Film 1562961