Charlotte Ellen SMITH / EIGHTEEN

[Merry]

Would I have to go to the register office in Banbury, or could I order a copy from my local office?

I don't think Banbury register office still exists.

FreeBMD give info on where historic records are held and they have this to say about Banbury district records:

Registers currently held at : Oxfordshire, Warwickshire (Warwick) and Northamptonshire.

So, you would first need to phone one of these offices and ask whether they, or one of the other two offices, hold the marriage certificates for Banbury reg district for 1861? Hopefully they will know the answer (I've had reg offices deny holding records and each in turn state one of the others has them!). Once you know who to approach, you need to provide all the info you have about the marriage along with your payment to the correct office. Some local offices have online databases and ordering systems, but this area doesn't seem to have that, so you will probably have to write a letter, enclose a cheque and maybe an SAE too.