Everything unique value would need to be counted Exept Rehab and defects so,

example:
OS14557 Rehab
OS14557 Rehab
OS75009 Ready
OS02221 Defect
OS75565 Ready
OS75565 Ready
OS11155 Project
OS16234
OS56643 TBC

So in this example I would want a value of 5 to be returned.

This being so I thinnk I've sorted out a work around

=SUMPRODUCT(((B3:B850<>"")*((K3:K850="Rehab")+(K3:K850="Defect")))/COUNTIF(B3:B850,B3:B850&""))

by using this formula I can count the number of unique rehab + defect instances and then take it away from the total number of unique items. It's a little bit untidy but seems to work well.

thanks for your help guys.