....... a local source has one available but from a rover 25. Does anybody know if this will fit/work in my 45?
The actual fan assembly (including the outer casing/ducting) is not
the same between the two cars.
The part number for the motor/fan assembly for the 400/45/ZS is different from the part number for the motor/fan assembly for the 200/25/ZR, so I would think it won't fit (or will need modification to make it fit - the actual motor within the assembly may well be the same, but whether it can all be easily dismantled and refitted to the 45 fan, I don't know).
For the correct one, you can try looking on eBay, or put out a parts request through one of the second hand parts finder sites such as PartsGateway
(there are many other such sites which can easily be found with a quick internet search).
Oh, and welcome to the forum