You will need two witnesses over the age of 16. As far as I know, the reg office are not allowed to supply them, so either think about two people who you could ask, or grab people from the street on the day!
For what sounds like a fairly private couple I imagine the latter option will not really suit; but you will need to think about whats worse, going into a pub or cafe and asking a couple of strangers, or having two people you know there.