Hotpoint WD860 - F-04 error?

hi, my hotpoint washer is showing an error F-04 just after i start the machine, i have read a few posts that it could be blocked or that the pressure switch could be jamed.

i have cleaned out the 'tennis ball' ( found £1.12 & several hair clips ) and i have checked the pressure switch with a multimeter ( 4 contacts with no pressure 2 are made and 2 open, when you apply pressure the 2 open ones are now made and the others open ) but i still get the same error.

any ideas?

thanks in advance Stu