Our dual flush cistern is becoming increasingly harder to flush as the push button is getting stiffer and stiffer to operate. I have looked inside and can see no obvious problem, can you help ? David

As with many of these things, it's fairly hard to diagnose without actually looking. Chances are that it's either a build up of limescale or some other substance, a substandard spring that is rusting, or something slowly becoming misaligned over time (such as a washer inside the unit). You could try putting some oil down the sides of the mechanism, but in many modern dual flush toilets the sealed units are very hard to maintain; sometimes you can find a replacement 'trigger' in DIY stores, but unfortunately the last time I came across a problem like this I ended up replacing the entire cistern after many fruitless hours of replacing washers, cleaning and poking.