Unable to remove fan on worcester bosch 28i rsf?

Have new 87161216810 fan assembly ready to replace old fan on worcester bosch 28i rsf. I've removed the four plate screws, mains and sensing tubes as described in section 15.3(e) of installation and servicing instructions, also loosened flue ring just in case but the fan will not come away from the flue pipe. Cannot see any other screws, etc, & reluctant to use excessive force. What's the knack for removing it?