A family of Microsoft relational database management systems designed for ease of use.
Union queries get their field names from the first subselect. In other words, the fields in your query will be [Item] and [Purchase Price]. You should be able to use either
SELECT Labour.Item, Labour.[Purchase Price]
FROM Labour
UNION ALL SELECT Materials.Item, Materials.[Current Value]
FROM Materials
UNION ALL SELECT Prelims.Item, Prelims.[Current Value]
FROM Prelims
UNION ALL SELECT AllInRates.Item, AllInRates.[Total]
FROM AllInRates
ORDER BY Item
or
SELECT Labour.Item, Labour.[Purchase Price]
FROM Labour
UNION ALL SELECT Materials.Item, Materials.[Current Value]
FROM Materials
UNION ALL SELECT Prelims.Item, Prelims.[Current Value]
FROM Prelims
UNION ALL SELECT AllInRates.Item, AllInRates.[Total]
FROM AllInRates
ORDER BY 1
Note, btw, that the original query you posted was invalid: you cannot have semi-colons inside the SQL.