If your mother's place is a borderline case (ie many people there or is very small) then it is probably a good idea to obtain one from your local council. Here's a link for the service in Scarborough (you will need to google for your local council)
http://www.scarborough.gov.uk/Default.aspx?page=8543
If though for example there's just the two of you living in a 3 bedroom house I wouldn't bother.
