I am considering taking my 3 children on holiday in August for a week, I have custody of them and they see their father who I am divorced from at weekends , he thinks I need written consent from him to do this, Is he correct?

It can depend on what was written in your divorce agreement and where you are going, but often yes, written consent is required... especially if you are going out of the country.
If the concern on his part is missing his weekend with them, perhaps you can arrange for him to have that time made up over a school break for example.