I'm not sure if they have a "quickie"/15 min rate. I think it's a minimum of 30 mins at £40. That's the basic price though and for OWO it'll be an extra tenner.
It is as easy as you make it sound although there may be no girls available at the time you walk in so you might have to wait in the waiting room (which is the same room as the reception). They will give you a rough idea of how long you'll have to wait for each of the available girls.
You can of course call and book someone in advance. You can also call when you're very close by/outside to check that there are girls currently available (so you don't have to go in only to be disappointed/have to wait).
Do note, although I hope it's quite obvious, that the parlour having a good reputation doesn't mean all that much because it really depends on the girl you see.
