Conditional INSERT INTO statement in postgres

That specific command can be done like this:

insert into LeadCustomer (Firstname, Surname, BillingAddress, email)
select 
    'John', 'Smith', 
    '6 Brewery close, Buxton, Norfolk', 'cmp.testing@example.com'
where not exists (
    select 1 from leadcustomer where firstname="John" and surname="Smith"
);

It will insert the result of the select statement, and the select will only return a row if that customer does not exist.