Hi Fina,
Yes the best way to find your Grans family name is to get your mothers birth certificate. It should have your Grans maiden name, your grandfathers name and occupation and their address at the time of the birth.
The marriage certificate will also show your grandparents ages,(full means over 21) their occupations, names and occupations of the respective fathers and names of witnesses.
From the ages on the marriage certificate its easy to work out their birth year, then you can look for their birth certificates.
If you want to trace the family tree these sites might help.
www.familysearch.org (Morman site)
www.freebmd.rootsweb.com (Births Marriages and Deaths)(not a complete listing but they are getting there!)
You don't need to register on either and the information is free.
www.1837.com is a pay-per-view site, and needs registration, but lists all Births, Marriages and Deaths registered since 1837.
www.ancestory.com has loads of info and you can have guest membership (free)
www.cyndislist.com - thousands of links to search sites.
If there's a Geneology society or group locally to you they could probably give you pointers too.
Wish we could give email address here I'd love to help you further!!!!
Hope this helps, and hope you find what your looking for. Good Luck!!