Potterton Puma 80 - Fan need replacing - Safety Question?

I have identified that the fan assembly on my Potterton Puma 80 needs replacing. I have found the item and can get it delivered. I am fairly happy about the replacement itself but I need to check afterwards that there are no leaks from the flue? How do I do this? Is there any other safety measures I should take to ensure that the job is done properly (ne seals etc?)

thanks