Depending on the timing of your proposed meet, as well as duration (are you thinking about an hour or so.....or an overnighter?), maybe you could try dayuse? They have rooms available during the day (obviously) but often have time slots stretching to 10 or 11pm.
A while ago I used the Millennium Knightsbridge. The rooms are tiny but perfectly functional, and the location is superb, being just a few mins walk from Knightsbridge station and Harvey Nicks as well as Hyde Park.
Let us know how you get on