--not sure what your datatype is for id, but:
declare @UserId int;
declare @email nvarchar(255);

--set @email to email you want
--then set @UserId to the email (below)

set @UserId = (
                        SELECT Top 1 id
                        FROM MainStreet.dbo.leads
                        WHERE email = @email
                        ORDER BY id desc
                    ) 

--then change your code at the end
AND ncb >= 1
AND ncb <= 5
AND id = @UserId

SELECT qid, postcode_area, vehicle_group, premium, lic_years, ncb, proposer_age, email, created
FROM Mainstreet.dbo.leads l
---------------
--try this join
inner join (
			SELECT distinct id
			FROM Mainstreet.dbo.leads
			) le
on l.email = le.email
---------------
WHERE created >= '01 January 2012 00:00:00.000'
AND created <= '06 January 2012 23:00:00.000'
AND scheme_id = 'PCH3'
AND proposer_age >= 21
AND proposer_age <= 29
AND lic_years >= 1
AND lic_years <= 5
AND ncb >= 1
AND ncb <= 5
--Remove the below AND and sub-query
--AND id = (
--			SELECT MAX(id) AS Expr1
-- FROM Mainstreet.dbo.leads le
-- WHERE(le.email = l.email)
--)

SELECT qid, postcode_area, vehicle_group, premium, lic_years, ncb, proposer_age, email, created
FROM Mainstreet.dbo.leads l
---------------
--try this join
inner join (
			SELECT distinct id
			FROM Mainstreet.dbo.leads
			) le
on l.email = le.email
---------------
WHERE created >= '01 January 2012 00:00:00.000'
AND created <= '06 January 2012 23:00:00.000'
AND scheme_id = 'PCH3'
AND proposer_age >= 21
AND proposer_age <= 29
AND lic_years >= 1
AND lic_years <= 5
AND ncb >= 1
AND ncb <= 5
--Remove the below AND and sub-query
--AND id = (
--			SELECT MAX(id) AS Expr1
-- FROM Mainstreet.dbo.leads le
-- WHERE(le.email = l.email)
--)

SELECT l.qid, l.postcode_area, l.vehicle_group, l.premium, l.lic_years, l.ncb, l.proposer_age, l.email, l.created
FROM Mainstreet.dbo.leads l

CROSS APPLY (SELECT MAX(le.id) AS id 
                FROM Mainstreet.dbo.leads le
                WHERE le.email = l.email) as le2

WHERE l.created >= '01 January 2012 00:00:00.000'
AND l.created <= '06 January 2012 23:00:00.000'
AND l.scheme_id = 'PCH3'
AND l.proposer_age BETWEEN 21 AND 29
AND l.lic_years BETWEEN 1 AND 5
AND l.ncb BETWEEN 1 AND 5
AND l.id = le2.id