It's dependant on several factors, what is the wage in your area? the size of your business, what can you afford to pay him/her?
You could pay from £25k to £40K depending on all the extenuating factors, such do you have a small salon or 20 rooms!. Work out what you can afford to pay them and make sure that's also pretty close to what they're worth, you can work in a commission structure based on the annual or bi-annual performance of the rest of the team if you want to create a base salary with commission. A good salon manager is hard to find, care for them well. Don't make the mistake of simply giving him/her a small pay rise from their therapist salary that can be highly demotivational.