select (

-- all udprns used on non-deleted prospects and customers >>>
(select count(udprn) from (
select udprn from prospects join addresses on addresses.id = address_id and deleted_at is null and udprn is not null
union all
select udprn from customers join addresses on addresses.id = address_id and deleted_at is null and udprn is not null) as tbl1)

-- minus >>>
-

-- unique udprns used on non-deleted prospects and customers >>>
(select count(distinct udprn) from (
select udprn from prospects join addresses on addresses.id = address_id and deleted_at is null and udprn is not null
union all
select udprn from customers join addresses on addresses.id = address_id and deleted_at is null and udprn is not null) as tbl2)

) as tbl3