Hi Lynnck,
It's always useful if you tell us which records you've looked at when you say 'I have been unable to find a birth/christening for Sarah'. Saves us wasting our time looking in the same place(s).

If you've looked on the IGI then you will be doomed to disappointment because it has no records for Boston St Botolph 1803-1812 inclusive, and possibly not for part of 1802 and 1813 as well. A bit strange really because according to the LDS catalogue film notes (number 1542030) the baptism registers for that period exist, and the IGI have baptisms for the years either side.
Sarah's age and place of birth are consistent on the 1861, 1871 and 1881 census, so I would say that the chances that she was baptised at Boston St Botolph are fairly high. Film copies of the registers for c1807 are available to view at Lincolnshire Archives
http://www.lincolnshire.gov.uk/section.asp?docid=28375&catid=6726Society of Genealogists in London
http://www.sog.org.uk/or can be ordered in at your local LDS FHC.
http://www.familysearch.org/Eng/Library/FHC/frameset_fhc.aspThe almhouses where Sarah was living in 1881 were almost certainly the ones erected by Sir Thomas Middlecott (or at least his charity) at what is locally known as Newton's Corner. (Check the previous census page for the Newtons!!)
From the 'Poorhouse, Poor Law etc' section for the parish of Skirbeck from Lincolnshire Genweb
http://www.rootsweb.com/~englin/"In Skirbeck Quarter, ten tenements for the residence of poor people were built on the site of Spital House, which was founded in 1625 by Sir Thomas MIDDLECOOTT."
Quite a bit about Sir Tom on the web if you google.
The almhouses were demolished in the 1960s (possibly early 1970s). Precise location of them
www.streetmap.co.uk search for Wyberton Low Road (street)
Top centre square, Middl Cl is just to the right of the green A16. The almhouses actually fronted on to London Road which is the white road running parallel to the river and which then turns yellow after it crosses the A16.
By the way, don't confuse the parish of Skirbeck Quarter with the parish of Skirbeck, they are on opposite sides of the river and before the Swing Bridge was built to take rail traffic to the docks it was quite a walk from one parish to another. Don't know what happened when the bridge was originally built but certainly way before the 1950s pedestrians and cyclists were using it as a short cut across the river. You just had to be careful that your bike wheels didn't get stuck inbetween the planks of wood on the bridge if there was a train due! (Sorry, going a bit off topic there, but although I know Sarah was alive when the docks opened in 1884, I don't know if she would have had a chance to walk across the Swing Bridge.

)
Pam
Edited because I realised I'd given incorrect URL for LDS FHCs.
Edited again on 11th July because I realised that the link to Lincs Archives was also wrong.

Don't know what happened.