From your Home page, Clients Letters doesn't work because you have the link reading:
http://www.eventdiscos.co.uk/client_letters.html
instead of:
http://www.eventdiscos.co.uk/clients_letters.html
Other Entertainment works, but I believe that is all.
The links look like they should be good. Are you sure you have saved and published the other pages?
Did you save and publish:
Quote page = quote
Questions page = questions
Venues & Clients = venues_and_clients
Contact Us = contact_us
Terms and Conditions = terms_and_conditions
notice all lower-case letters, no spaces, nothing else added such as '.html' when saving/publishing.
The info above reflects your links but it's not finding your pages.