Thanks for the help! Got it working with:

SELECT tblErrors_ER.ErrorID, CASE WHEN Count(tblPolicyNumbers_ER.PolicyNumber) = 1 THEN MIN(tblPolicyNumbers_ER.PolicyNumber) ELSE 'Multiple' END

FROM tblErrors_ER

INNER JOIN tblPolicyNumbers_ER ON

tblErrors_ER.ErrorID = tblPolicyNumbers_ER.ErrorID

GROUP BY tblErrors_ER.ErrorID

Thanks

Carl