There are a few discussions on overnights, the majority opinion is that they are a waste of money, I've done three and I'm tempted to agree with them now. Twelve hours is a long time to spend with a pro$$ie and there isn't much you can talk about. On the plus side there is absolutely no rush. You get together, chat for a while and get the brains into gear before round one; eat,drink and be happy before round two; sleep and be ready for round three in the morning; or whatever your plans, stamina, prowess may be?
My suggestion is that you write out a reverse booking detailing your plans/hopes for the night, location and a few preferences about the type of pro$$ie you would like (age,size,etc) about two weeks before the night in question. Depending on what you are looking for set your price around £400 and see what offers come your way. Bear in mind you will be expected to pay for all food and drink, plus the hotel.