How do I change the fan assembly on Worcester i28?

Hi,

There's a fan problem with my Worcester i28 gas boiler. When it is supposed to turn on heating, it starts the fan and then ignites, but what I have a fan moving a bit and the stopping. It doesn't spin as easily as it supposed to so I assumed it is faulty. I've looked up for a part number online and found it. But before buying I wanted to make sure I can get the faulty one out. Undone 4 screws holding fan assembly but it still won't come out as it is connected with an exaust pipe. I have also loosened the tie but it is still not coming out. Any help would be appreciated. Many thanks in advance.