Ok so this gym is not actually by Swiss Cottage tube but in fact is located just a few minutes walk north along the Finchley Road, by the Finchley Road tube station, in the 02 centre. (Map is wrong - location is at the intersection of rail track and the green road).
Esporta is a pretty decent members-only gym, newly refurbished and with a high standard of equipment. It's kept in good clean order and there are plenty of classes - aerobics, spin, pilates, yoga. The complex includes a spa, a hairdresser, a pool, mixed sauna, mixed steam room and then gym spread over two floors. There are plenty of cardiomachines, fixed weights, free weights, and workout mats. Even at the busiest times the gym never feels crowded.
If you're interested in joining do negotiate hard for discounts with the membership staff.
Four stars not five as the place can be feel like it's overrun by kids at the weekend (although they can't come into the gym itself).