Failing to successfully add a ribbon gallery to a PowerGallery product's detail page

I am failing to successfully add a ribbon gallery to a PowerGallery product's detail page. I start with a functioning store. Test products are entered, and their detail pages show fine. I have posted the problem previously, but received no advice.

To add the (PowerStore) PowerGallery to one of these items I do the following:

1. I upload the file wa_evaluate.php to the directory (store) that holds all my PowerStore files and folders. My Power Gallery folder is also located in this folder.
2. The instructions say, "Open the page that will display your PowerGallery from a database" so I open the Products_Detail.php page in the store's root directory.
3. I find the line, <div id="ProductLongDesc"><?php echo $row_WADAProducts["ProductLongDesc"]; ?></div>
4. As per instructions, I change it to:

<div id="ProductLongDesc"><?php require_once('wa_evaluate.php'); ?>
<?php echo wa_evaluate($row_WADAProducts["ProductLongDesc"]); ?></div>

At this point - each and every time - the site 'breaks'.

5. Carrying on regardless, I create the 'Ribbon + Lightbox for PowerStore (light)' gallery.
6. I select a product, click to update it, and in the source code view of the Long Description editor area, I paste the code from the PG 'Add to site' > 'Add to PowerStore' area:

<input type="hidden" name="powergallery" id="1" theme="light" value="/home/users/uks66722/html/206bones.co.uk/store/gallery/galleries/ribbon_lightbox_powerstore/index.php" />

7. Save the changes.
8. View the disaster that is the product detail page, as I have on several previous goes and on different PowerStore installations.

I can see the very top of the inserted gallery on the product detail page along with the error message:

*Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/users/uks66722/html/206bones.co.uk/store/Products_Detail.php:1) in /home/users/uks66722/html/206bones.co.uk/store/WA_eCart/WA_eCart_Definition_PHP.php on line 480

I have done the usual 'Cannot send session cache limiter - headers already sent' checks.

I have seen other peoples' successful implementations, so am aware that I have messed up somewhere. Please could somebody write out the instructions on how I should be implementing the ribbon_lightbox_powerstore gallery for a product, or suggest where I am going wrong?