Hi there I changed to no slash in front of webformmailer, and no difference. I've also tried a complete name including http, ../directory etc.
Yes, the webformmailer.php is in the same directory as the form.

<form id="bid" name="bid" action="webformmailer.php" method="post" >
<input type="hidden" name="form_order" value="alpha"/>
<input type="hidden" name="form_delivery" value="hourly"/>
<input type="hidden" name="form_format" value="csv"/>


<span id="sprytextfield1"><label>Your email

<input type="text" name="email" id="text1" />
<span class="textfieldRequiredMsg">ie yourname@domain.com.</span>
</label></span>

<p>
<label>Subject
<input name="subject" type="subject" id="subject" value="Bid for Gallerie Diamante artwork" size="40" />
</label>
</p>
<p>
<span id="sprytextfield2"><label>Artwork code

<input type="text" name="artwork" id="text2" /><span class="textfieldRequiredMsg">Please enter the artwork name.</span> </label></span>
</p>

<p>
<span id="sprytextfield3">
<label>Bid amount CDN

<input type="text" name="bid" id="text3" />
<span class="textfieldRequiredMsg">In Canadian Dollars.</span> </label> </span> </p>
<p>
<span id="sprytextfield4"><label>Your phone number for bid confirmation
<input type="text" name="phone" id="phone" />
<span class="textfieldRequiredMsg">ie 250-717-5304.</span> </label></span>
</p>
<p>
<span id="sprycheckbox1"><label>
<input type="checkbox" name="rules" id="checkbox1" />
<span class="checkboxRequiredMsg">Please check the box.</span>You confirm that you have read the <a href="/diamante/howtobid/howtobid.html" target="_blank" class="style1">bidding rules (opens a new window)</a> and agree to accept them.</label></span>
</p>
<p>
<label>
<input type="radio" name="radio" id="newsletter" value="newsletter" />
Please add me to your newsletter (sent via ConstantContact). I understand I may opt out at any time.</label>
<a href="/diamante/privacy/privacy.html" target="_blank" class="style1">Please see our Privacy Policy for more information (opens a new window).</a></p>
<br/>
<input type="submit" value="Submit" />
<input type="reset" value="Reset" />

</form>

<form action="" method="post"><input name="email" type="hidden" value="juliatrops******.com" />
<input type="hidden" name="redirect" value="../diamante/contact/thankyou.php" />

</form>

There is a captcha snippet in between the form method post.