I have now managed to find it and change it. I hope this will help any others that need to replace the resistor.
First where is it? On my 59 plate, RHD Fiesta Mk7, it is on the passenger (near) side but hard to find.
How to get at it? I had to remove the trim below the glove box. There are 2 black pull-out toggles that secure it and then it slides foward and can be removed. Next, I had to remove the trim that runs fore and aft, on the side of the centre console, just above the carpet. There is one black plastic plug (oddly has a phillips screw head but doesn't unscrew) that pulls out. If you can, pull the middle out and then it comes out easily. If you can't get the centre out it will pull out but care is needed. Then there are clips at the rear end of the panel (below and a bit back of the gear lever) taht also pull out. Take care to ease them out.
Note that you do NOT need to take the glove box out.
Once the panel is off, you can see the resistor. It is well forward, and hard to see (let alone to get at). This shows where it is and this shows it a bit better . This last one shows it in close up .
Once you can see it, there are just the 2 torx screws to remove the resistor, unclip the connector block connect a new one and then put it back - replace the trim and it's all done.
It sounds simple but is a struggle to get in there and be able to see what you are doing.
I hope all this helps some other Fiesta owner. Given the history of faults on this component, it seems odd that Ford continue to use and to put it in such a difficult position.