tricky,

If you turn on db tracking,  it will track all responses in a cforms WP table, with a unique  number (counter) associated. The number shown on the Tracking page is just a local counter showing the current # of save form submissions, but it's be easy to additionally show the actual message ID.

This however requires the db tracking feature to be turned on.

Would that do?

Showing it online is not an option, since it'll be determined after the form is submitted and just before the email gets sent.

Take a look at cforms.php (line 470-480 & 953-960). After this line:

$wpdb->query(substr($sql,0,-1));

you could add:

Then include the submission ID in the email, change (~508 &  +1045):

e.g., to: 

I tested this, it works and would be how I would solve it.

Of course you could also add the ID in the tracking report. In cforms-database.php, change line 190:

<li class="col0"><?php echo $i++; ?></li>

to:

<li class="col0"><?php echo $entry->id; ?></li>

This will replace the generic counter (always starting from 0) with the actual submission ID. 

I'm using a cformII as a petition and would like to display a counter on the page where the form resides to show people how many people have submitted so far and hence that they've made a difference by submitting!

In plain english I'm after some code to put above the form to show:

count wp_cformssubmissions where ID="1"

I think i'm along the right lines but my php skills are virtually non-existent.

Thanks in advance.

...or you could have used the built-in function call:

with X being '' nothing or 2,3,4...

you can, per the Help! page: {ID}