How long does a company have to reply to a complaint?

I sent a letter of complaint by Special Delivery to a travel company on 22nd May and have heard nothing. How long do they have to reply and what is my next step if they don't?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).