From experience, thermostats on cars lose their accuracy over time without you fully realising, and I'd advise changing yours if they coolant is due anyway and the car is over 10 years old.
Tell-tale signs are the gauge reading slightly higher on the motorway than town driving.
To make sure you get the right part, go to your Ford dealer. Ford's definition of the correct temperature varied a lot on the push-rod engines between the 1980s and 1990s. I know on the Ka they should all be the same but I'd rather you be on the safe side. These thermostats are relatively cheap as they don't come fused into the housing, as many new ones do, so you should get change from a tenner.
I had an Escort once that I ran from 13 years to 19 years. The gauge characteristics were as described, fell into NORMAL each time and rose slightly on motorways and this never changed, but when I fitted a new one the difference was immense and the car heated up a lot quicker.