I think your competition are providing a much more attractive offering, which could be part of the reason why you have no customers.
website spell checking: $50 one time payment
http://www.inspyder.com/products/InSite/
flat fee based on number of pages:
http://www.texttrust.com/TextTrust_Express.html
IMHO, your per mistake billing model means that the customer cannot be sure what the final bill will be. They will read "£5 per mistake" and instantly bail out. They might not even continue reading to your next sentence which provides details of the fixed cost first time offering.
Other issues:
- site design very outdated
