I'm attending a soirée (ie pissup) in Baker St next Saturday. I need a hotel and somewhere vaguely safe to leave the motor. I'd rather keep the cost to a minimum to be honest as it's just a bed for the night, nothing more.
The Churchill Int on the west side of Portman Sq has an NCP just behind it. Nice hotel and secure car park. 50 yds from the bottom of Baker St. (Also has good cigar lounge).
Selfridges Hotel, right at the bottom of Baker Street/Oxford Street. You ought to be able to get a discounted room from thisislondon.com and their ilk. Nice place, not terribly expensive.
Used the Royal National in Russell Square for almost feck all. Only a 3 star but has a secure carpark underneath and is unlikely to be full - it's got 1600 rooms.