agreement in principle or credit check first?

Hi all,

Could someone please just let me know how this works.First time buyer.In order for an offer to be taken seriously/give us more bargaining power,I intend to get what I believe is a mortgage " agreement in principle".

Q - will the Mortgage company do a credit check before agreement in principle, or do they agree in principle based on basic facts such as income,deposit etc etc,then start the heavy stuff,pay slips,credit check,references etc once you actually apply?

Thanks - Rob