The information comes from several different sources including your application form, any past accounts you've had with that lender and anything recorded on your credit file.

Your application form

This will have your personal details, plus information about your other credit commitments. It'll also have details about the property you want to buy.

Go through this with a fine toothcomb. It's not unusual for a mortgage adviser to have taken down your details on the phone as they go through the application form. One slight slip, such as a "£2,000" salary rather than a "£20,000" one, can immediately kibosh any application and possibly future ones too.

Any past accounts you've had with that lender

If you're applying for a mortgage with a lender that you've had dealings with in the past, for example if you've had a credit card with that lender, it'll use this information to add to what it knows about you.

Your credit files

The three credit reference agencies - Experian, Equifax and TransUnion (formerly Callcredit) - compile information, allowing them to send data on any UK individual to prospective lenders. All lenders use at least one agency when assessing your file. This data includes court records, fraud data and any information about any credit cards, utility contracts or bank accounts you've got.