Hi
Sure someone will correct me with this reply if I am wrong

The GRO index ref is needed if you apply to the main office in Southport. There is a link to it on this Site. You have to pay them more if no index ref.
Personally I would apply to' Belper Derbyshire Register Office' for the Certificate as it may be cheaper and 'should' arrive quicker. Also you won't need the GRO ref just names, event, year etc.
If you go on
www.genuki.org.uk/ there is a list of register offices on there.. might even have some other information that could help.
I have only sent for full certificates myself so can't help with that but you need all the help you can with this hobby

so would suggest the full one..
Tip : A death certificate I obtained when I started searching appeared to be no help at all. Thought it was a waste of time and money!! However a few years later and finding more information it has turned out to be a real help. Not that you are going to take that long of course

they did not have all these great genealogy sites when I started.
best wishes
Reyz