Toilet flushing?

When I flush the loo, it takes forever to refill - almost as if the water is being replaced so slowly that I can't hear it refilling.
The loo is a domestic one, 5 years old and made by Twyford. It has a push-button top on the cistern which you can't just simply lift off; you appear to have to unscrew the outer edge of the flush button to lift the top of the cistern off to see what's wrong inside.