Good question.  Some councils and local authorities can be quite funny.  I rent with two other people, we have a joint tenancy and a joint bank account for the rent so I would say we are quite clearly not an HMO!  However if we all paid our rent seperatly to the landlord under the terms of our joint contract would it still be as clear cut....
We are three people living our own lives sharing communial parts of the property.  The key IMHO is the fact we have a joint contract.  However I have head of councils trying to apply all the HMO rules to 2 bed flats that are rented to sharers!!

Is there a decent definition out there?