Solved.

After trying various Master Dimension expressions, including the two below which should work:

    =If(NOT IsNull([Lease Ref]) OR Len(trim([Lease Ref]))<>0,'Leased','Not Leased')

    =If([Lease Ref] & '?' = '?','Not Leased','Leased')

I eventually tried:

    =If([Lease Ref] & [UNIT.Unit Ref]=[UNIT.Unit Ref],'Not Leased','Leased')

This works! Haven't got the faintest clue why it does when the 2nd one didn't but, who cares.

Thank you all for your help.

Regards,

Peter