Hi,
I've had a look through St Peters parish in the 1841 census, couldn't spot William or Sarah, but this entry may be relevant.
ref HO107 468-19 folio 561 page 3: Belmont House, St Peters (near to Calais Court)
James Patteson age 32 M S (male servant) born in county
employed in the house of John and Georgiana Thompson
I've no idea where else to look for William and Sarah, as servants they could be employed anywhere.
xx Carol