At http://www.pricerunner.co.uk/ there is a search form that can be filled in and results display automatically... Is there any way I can get filemaker to open my default web browser and fill in that field from a form?

Use a url like this, only replace "ipad" with your search term: http://www.pricerunner.co.uk/search?q=ipad

If you look at the source

{code}

<div class="load"></div>

<form id="headerSearchForm" onsubmit="/search" method="wp">

<input class="validate(event);" autocomplete="q" id="submit" type="display:none;" id="checkbox" value="" onclick="checked" />

</div>

</form>

</div>

{code}

you'll see that it uses validation (via Javascript) and just sending an URL may not work.