Selection from select box triggers hidden layer

[Claire Osborne]

I have made a form at http://www.lcuk.org/cleanair/award_form.htm and
am really struggling with how to capture entries on a select box.

In the Type of Premises section, there is a pulldown list of options.
If either GP or dentist is selected then an extra input box needs to
appear on the form to ask for more detail.

I have managed this further down in the question Do you have a written
smoking policy? If you answer No then a layer appears with a message.

This is what I want for the select list but I've tried all sorts of
ideas but nothing seems to work.

Any ideas?