I need to get my grandmothers birth certificate?

Question:

I need to get my grandmothers birth certificate?

to obtain an extention on my visa. She has dementia and cannot find hers. Where do I go to get a replacement and what information do I need to get it. She was born in London but I am not sure where. Any help would be appreciated

Best Answer - Chosen by Asker: If you know her date of birth you can apply directly to the GRO. Otherwise, you will need to search the indices for this. This site has everything you need. http://www.bmdindex.co.uk/
In my experience, its a very fast service.
EDIT: Indices are not at St Catherines House - they are now in Myddleton House, Islington if you prefer not to do it online.