If you can get some penetrating lube similar to what you would on rusty bolts spray the offening link. This should help but only if the stiff link is due to rust or lack of lubrication. If the link was damaged it won't get better. In the U.S. we have Liquid wrench,SeaFoam, PB Blaster etc. you want something that is thin enough to seep between the plates. If you can soak a small rag with the penetrant and wrap it around the chain overnight. Then see if you can work the link in both direction by hand. Most of the aerosols will evaporate so the rag gives you a longer soak. If the link frees up spray it with a chain lubricant so it will stay flexible.