Hi T - one site to suggest is go to:
http://www.farmstayuk.co.uk/farmnew/index.asp and search the Northumberland area on the location map for SC facilities - quite a few will accommodate short breaks - check the links for the providers and they offer their contact details and tariffs for stays less than 7 nts. and there a few nice choices to look at.
Also a trawl through the SC accommodation at :
http://www.visitnortheastengland.com/site/home for the NE England T/Bard should give some good resu;ts. There is also SC accom. around the Northumberland National Park - accom. info link:
http://www.northumberland-national-park.org.uk /VisitorGuide/Visiting/Accommodation/default.h tm and follow the links for details.
Hope this is of use, and have a nice stay..