SELECT Query Including JOIN And CASE Expression

An error is entered into the table, across two tables - tblErrors_ER and tblPolicyNumbers_ER - each error generates a PK (ErrorID) and can have any number of policy numbers which will be referenced by its own PK but linked to each error by its FK (ErrorID).I want to display each error in a Gridview in ASP.Net - columns included will be ErrorID, ErrorType, DateLogged from tblErrors_ER and PolicyNumber from tblPolicyNumbers_ER.If an Error has more than one policy number I only want to show the error once in the GridView with the word MULTIPLE under policy number.

tblErrors_ER
---------------
CREATE TABLE tblErrors_ER
{
ErrorID int,
ErrorType varchar(255),
DateLogged datetime,

[code]...

I have changed the Count(*) to Count(tblPolicyNumbers_ER.POlicyNUmber) which gives me the same undesired result as above. I have also left it as Count(*) and the entire CASE expression within the GROUP BY statement as suggest above which generated an error saying I can not use an expression in a group by clause.

If I leave Count(*) = 1 where it is in the original SELECT statement but swap the = for > then something happens, close to what I require but not as intended. It returns:

ErrorID ErrorType DateLogged PolicyNumber
---------------------------------------------------------------
1 Test 08/08/2012 Multiple
2 Test 08/08/2012 Multiple

this would suggest the original syntax is close to being accurate but I can not get it to work.