This one may be a little difficult to explain, but here goes. I have started to install some scripts from php jabbers just lately, and most go without issues. but after installing this car rental one which i have made into a motorhome rental script i get an issue right at the end after filling out all the details. Aparently PHP Jabbers tell me it is the About the gap - it seems that the height for this container "imContentContainer" is calculated automatically. When the booking form is opened the div min height get set to 3000px and it stays like this when the form is completed. This is something that your template does and the script does not have any control over it. 

So if you would like to test this out start a booking off at http://test6.in-derby.com/book-online.html dont fill out the form you can bypass it all but tick agree to t&c and tick cash and you will see what i mean. the form should return to the form and not create a big gap on the webpage.