I have done a few open days/charity events/health fairs & tasters over the years, but never with the expectation of making any money out of it. I have done a local health fair, where people were encouraged to buy a £5 voucher at the door, to use on the therapy of their choice. Any therapist in the room, who was handed vouchers, was able to submitthem to the organiser at the end of the day. Each therapist was expected to spend 15 minutes with each voucher holder. I submitted 24 vouchers at the end of the day - I was exhausted, but thoroughly enjoyed the day - especially being with other therapists from different modalities. I cleaned 5 regular clients from the event, who I still see 18 months later.
At charity events I have done, I have paid for 'stand space' and have not expected payment, but by handing out leaflets I have gleaned extra clients and covered any 'loss' I may have made.
To answer your main question, yes, £15 seems reasonable. Ifthe event is acharity one, you may prefer to talk to the organisers about refund oftravel expenses and a donation for products used, rather than an hourly fee - they may insist on paying you, if they think you are willing to meet them half way, in which case you could 'suggest' an hourly rate! Good luck - and keep your leaflet supplies well stocked!