this is not a problem, this is the way captcha is supposed to work.
the form is only supposed to submit if the correct captcha code is entered.
when i view source of your page, the image tag looks like this:
<img class="Captcha" alt="Security Code" src="WA_ValidationToolkit/WAVT_CaptchaSecurityImages.php?field=Send_enquiry_group_Security_code&transparent=false&font=Fonts/MODERNA_.TTF">
try replacing that with:
<img src="../WA_ValidationToolkit/WAVT_CaptchaSecurityImages.php?field=Send_enquiry_ group_Security_code&transparent=false&nois efreq=25&gridfreq=50&font=Fonts/MACTYPE.TTF" alt="Security Code" class="Captcha" />