I think your request has been moved to the right page now
I was going to suggest that you had a look on the Sussex Family History Group site to see if anyone else was researching the Polhill name but if they are they're not on the public pages
http://www.sfhg.org.uk/You could have a look at the Surname Interest board here in Rootschat (link at bottom of page)
There are a few burials recorded on Family History Online for Sarah Croft in Sussex. Although pay-per-view it's well worth the small outlay.
http://www.familyhistoryonline.net/I think it would be wise to search in Kent as well as in Sussex. However I did find on the National Archives Site the Bastardy exam (held at the ESRO) 'Sarah Polhill charges Alexander Partridge of Whatlington, labourer', so now you know who Thomas's father was!
The A2A (Access to Archives) site gives 'Mary Polhill charges John Pettitt of Hawkshurst, Kent' 14/4/1767.
A little earlier in Ewhurst, 24/4/1764 - Warrant to arrest Putative father's. 'John Rummins of Westfield, servant, father of unborn child of Mary Polhill, singlewomen'. Mary herself was examined on the same day.
Looking at the full reports held at the ESRO might give you some information. If you can't go yourself, perhaps you could contact them to see how much they charge for a look-up,
jane