Hi Helen,
Do you have an Instagram or twitter account? Both of these are also really useful in marketing terms and can cost you nothing but your time. You can pay for advertising if you wish, like with Facebook, but Instagram (especially) is really the only social platform that doesn't hide away your posts away like FB does, or like Twitter does just from the sheer number of tweets going up every second.
Both Twitter and Instagram are very effective when using hashtags, try posting a few tan-related photos on instagram with lots of relevant hashtags, even using geographic ones to your local area, beauty-based ones, tanning ones, etc. and you'll be surprised how over time you will start to build up a following, some of which may hopefully also be local to you. You can then use those platforms to run special offers and you may get some bookings via them in the future. Just another one to add to the list.
ps. I fully agree on all the comments regarding price, £10 is certainly too cheap and may be questioned by a lot of potential customers. £20-£25 minimum depending on location/competition is more on par.
One more thought... have you considered offering retail products (tan extenders, body scrubs, etc.) as part of your spray tanning 'package'? That might give you an edge over other local competition if you can get some retail products at trade price from your preferred brand and then offer them as part of an overall package deal to each customer. It may also help increase your overall profit per customer too.
Best of luck with your business!
Al