Since your Mum and Dad have lived apart for more than one year (with your dad's consent) or, two years without your Dad's consent, your Mum can sue your Dad for Divorce - the above circumstances of non-cohabitation are the grounds your Mum would need to prove before the Court can grant the action for Divorce. She would start proceedings at her local Sheriff Court. You can find your local Sheriff Court at www.scotcourts.gov.uk
The Scottish Courts Service have also published some very helpful information on how to obtain a 'simplified divorce'. Your Mum should have no problem obtaining such a divorce, and they also provide links to the appropriate forms your Mum will need to submit. Form 49.73-B is the one she will need.
http://www.scotcourts.gov.uk/session/rules/cha pter49_11.asp