I may be a therapist, but I'm also a regular punter who likes going to salons for treatments.
Don't take this the wrong way....but if I visited you for a treatment, with a salon at 11deg...I wouldn't come back. I might sit with my coat on to have my nails done.....but a massage, or pedicure!!!!!! Never.
I won't be the only one thinking like this. You deserve a medal for managing to keep any returnees, just think how it would be if you had a warm welcoming salon, where people loved to visit you?...so much better.
Your landlady is costing you business...badly. either tell her you need to heat your space properly, or move elsewhere.
I tend to think if she is that shortsighted over heating....what next?