Hello Paul,
I believe I understand what you are trying to achieve. I am not sure if the following will help.
I have been trying to pass parameters to a page to play flv movies via a playlist.
The solution I found was to write php code and place it in the root of the web site in its own folder (htdocs/movies). For extra security I used this code just to test for the necessary parameters then to include a further php page that was stored in a none moodle generated folder in moodledata (moodledata/jk) . I was then able to use the moodle course option to "link to a web page", opening in a new window, to run my .php script in htdocs/movies, passing parameters in the url e.g. ?w=1020&h=576 etc. This then ran successfully. So it may be if you can take your form out of the Moodle lesson and link to it as a separate web page then the page will run correctly.
Good luck
Johnathan