Probably the best would be to link up with tour operators or hotels who want to offer the tours to their customers. If you can get them to actively promote your tours you can perhaps pay them commission for the number of people they have added to the tour. ie give directly incentives for them to do your selling for you!
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James Smith
Chartered Accountant
www.jamesesmith.co.uk

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Your indispensable guide to Small Business Bookkeeping, Self-Assessment & VAT

Surprised it has not been mentioned, how about making a facebook group to futher promote you're business. You make the group in you're companies name, attach a few pictures then start adding folk who have used you're business already. Advertise vouchers, special deals or discounts on you're facebook page, it is FREE!!

also get in touch with local travel agents and get you're name out there! they have justed started
walking tours in Glasgow. I bet it involves a tour around the pubs or discount food stores.
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You cannot give yourself the title of "entrepreneur" someone has to give it to you

Great guide to digital marketing:

http://www.seomoz.org/blog/the-noob-guide-to-online-marketing-with-giant-infographic-11928