Sorry, can't help with camping shop - unless Roy's of Wroxham have one! If they cannot get a new filter, they can still use the pump if they take out the old one, remove the white filter part and screw back the cap. Water won't be filtered but the pump will still work.
Try Jeckells Chandlers in Wroxham - the filters may also be used in boats.
Failing that it's Drayton Caravans, Heigham St, Norwich or Norwich Camping in Blofield.