Can't remember the name of a book....

[jean nurse]

I read a book whilst on holiday and can't remember the title... a humorous book about a family who start off in GB and for a joke forge a birth certificate for their son nicknamed Superman. Then they move to France and start forging more documents to help illegal immigrants and it spirals into quite a lucrative business amid chaos with the locals.
Does anyone recognise the plot and do you have a title and author's name?
Would be most appreciative. It was such a laugh. Thanks