Thanks Cronk,
That worked nicely but I had to put it in both halves of the Union: (what does the nz stand for?)
SELECT a.Letter, a.Organisation, a.Incumbent, a.Salutation, a.Address1, a.Address2, a.Address3, a.Town, a.County, a.Postcode
FROM [LEAF Member Churches] AS a
WHERE nz(Letter,0)=-1...