300 watt is one heck of an uplighter! Is it portable or fixed installation?
If portable note whether 2 core (if double insulated) or three core if there is an earth.
Basically you need to replace like for like. It will be heat resistant and most likely have additional fibreglass sheathing on the individual cores close to the terminations.
Best way, take a sample of the old to your supplier, making sure they are aware it's heat resistant. Heat resistant flex is usually marked as such along the outer sheath.