first do a compression test on all cylinders. a tester costs £10 at your local motorfactor, make sure you get a screw in one, not a push in one. if you get good even pressure on all cylinders that builds up quickly it tells you that the internals are doing what they should, and then you can safely say the problem is ignition or fueling.
next, while you have the plugs out for the compression test, you should check for spark on each one. if you have any doubts about the condition of the leads or plug then change them. compare the colour and quantity of the deposits on the plugs, if any of them stands out as different to the others thats a clue to which cylinder is suffering a fault., so is the colour. report back your findings and we'll diagnose it from there.
next on your list of checks is the injectors. the 1.6 suffers from injector failure a lot. if you search this part of the forum you will find many threads explaining how to diagnose and repair an injector fault. thankfully its cheap and easy.
to test the injectors use a multimeter to measure their resistance. there is a connector at the right hand side of the fuel rail (when looking in engine bay) if you disconnect it you'll see 5 pins inside. test on the connector that is on your left, use the centre pin as the earth, and the other 4 pins are each for an injector. measure the resistance for all 4, all should give a reading of 14-15ohms, if any are lower it means they need changing, anything lower than 13ohms should definately be replaced. a scrapyard is the best place to get injectors, they cost £5-10 each.