In taking apart a relative's Creda 48331 standalone cooker to replace the main oven fan I found the chassis hole instead of being sized for the fan-backing circlip & washer is actually clearance for the shaft and not circlip & washer.
That means you have to remove the circlip & washer before you can remove the old fan motor.
Even more fun was needing to unscrew both side panels & top hob, because the rear panel slips under them. Makes it more rigid re standalone but talk about slower to service - why not a bleedin hatch.
Now the replacement fan :-) It has a washer & circlip like the old, but the washer is press-fitted to the shaft. So you will need to cut the washer off with the thing bagged & taped in plastic re filings (or use a miniature puller). Then you can slide it through the hole and re-use the original brass washer with the new circlip.
This is sort of "jack car in air, lower engine & sub-frame, change spark plugs" :-(
Off to break a hacksaw blade and cut the washer off...