This should do it
<FORM ACTION="http://www.google.com/search" method=get>
<div align="center">
Find in this site:
<br>
<input type="hidden" name="output" value="myresults">
<input type="hidden" name="sitesearch" value="www.cableforum.co.uk">
<input type=text name=q size=10>
<input type=submit value="Search" name="submit">
<br>
</font></div>
</FORM>