Assuming that you have not got cable problems, if my experience is anything to go by, it's likely that the internal clutch operating mechanism is worn.

The clutch on my car got increasingly heavy to operate but the clutch operation was unaffected. Quite suddenly at around 140k miles it got much worse. The clutch was replaced and correct operation restored.

The old clutch showed considerable wear on the operating fingers to the extent that they were not being pushed squarely by the release bearing. The friction surfaces were still in good condition.

Having/having had both 1.0 and 1.4 Lupos in the family, both have cable operated clutches.

I've had the pedal bracket replaced on both of them, not a quick job; if the clutch is heavy, the bracket will fail. If the clutch action suddenly feels different, get it checked quickly, is my advice from experience,

The 1.4 still had a stiff action after new cable and pedal bracket; trusted independant and main VW agent advice was that clutch needed doing, daughter sold car at that point (now has a Fiesta, but misses her 1.4 Lupo dearly; low mileage Fiesta hasn't been trouble free).

Hutch,

You will need to get the clutch pedal assembly replaced. They wear after a time and the operation gets heavier and heavier until it will fail completely.

They're not the easiest thing to change as, if memory serves, the steering column, brake pedal box and part of the dashboard needs removing to access it.

Contact a few Indy garages or VW specialists for a quote. Main agents will charge an arm and a leg for this repair so it won't be cheap I'm afraid.