Sorry, okay you made the 3 or 5 door bit clear in your opening post.
In general, and with all these cars for sure, the offside will always have a high intensity rear fog light and the nearside will always have a reversing light, and means only one of each on each car, some other cars might still have 2 reversing lights and even 2 rear high intensity fog lights.
So having one of each is considered the norm and so for your mum's car to be fit for use in UK it must have a single reversing light it seems according to your MOT failure.
So, what to do now, well it looks like the rear high intensity fog light is fitted and working as if not it would have failed the MOT, which just proves that the reversing light bulb and the rear high intensity fog bulb are not located in the same point in the opposite bulb holder - which they did tend to be in past where rear cluster design demanded that be so due to lack of space.
You should rightly or wrongly assume that if the reversing light bulb is okay and its fuse is okay, from a list that I made up for my daughter's late 2009
Ibiza 1.4 SC, fuse 05 which is a mini fuse red 10.0Amp is the fuse for the reversing light. That means, when you pull off the fuse/OBDE trim/cover, the fuse you need to check is the 5th fuse in from the LHS and on the top row, you will see that the layout of that fuse box is - on the LHS, 3 rows of mini fuses namely 1>8, 9>16, 17>24 from top row to bottom row. Then 25>41 and 42>58 for the RHS group of ATO sized fuses - all numbering read from left to right it seems!
Next if need be, from Haynes 6R Polo repair manual, the reversing light switch is screwed into the front of the casing on the 02T gearbox (5 speed petrol models). From the picture shown, it appears to have 2 wires, so maybe unplug that lead and short these 2 wires together to see if that switches the reversing light on when the ignition is switched on, if it does then replacing that switch should sort things out - when you look under the bonnet make sure that there is no wiring damage showing.