Drupal is messing with my URLs

I'm building a D6 site which features Ubercart and CiviCRM (2.2). I want anonymous visitors to sign up for a membership via a CiviContribute page. I can get the CiviCRM profile included in the page to require visitors to have a Drupal user account before they can successfully sign up for a membership, thus ensuring synchronisation between Drupal users and CiviCRM contacts so that I can ensure that members get assigned an appropriate Drupal user role (giving them discounted pricing, etc.)

I can set things up so that when a non-logged in user hits the membership sign up page they get an opportunity to register or click on a login link. I can set the login link to http://mysite.com/user?destination=civicrm/contribute/transact?reset=1&id=1 which, if it worked, would take them to the login page at /user and upon successfully submitting that form they would then be taken back to http://mysite.com/civicrm/contribute/transact?reset=1&id=1, otherwise known as my membership signup page.

But, when I submit the login form, instead of being taken to http://mysite.com/civicrm/contribute/transact?reset=1&id=1 I get taken to http://mysite.com/civicrm/contribute/transact?reset=1, which is wrong.

I posted a query on the CiviCRM forum here, the response to which suggests that Drupal is actually stripping the trailing "&id=1" from my URL.

What can I do, if anything, to stop this from happening?

Thanks
Graham