Next door to The Hai Pieng restaurant, just up from Somtawin school, sort of opposite Soi 51.
Saw it the other day and thought they were selling stuff but have just been in and it's a gym. They were quoting 89 THB a day but there are monthly rates - I think she said 1,200 but I could well be mistaken.
Don't know whether that's good or bad. They seemed to have lots of equipment.
I'm gonna find out more tomorrow when I'm sober