Toilet Cistern won't fill (Villeroy & Boch)?

Help!
We have a very old bathroom suite with even older plumbing - the toilet is Villeroy & Bosh and for several months now has taken a long time to fill after flushing (it sounded like it was just trickling into the cistern). A couple of times the cistern wouldn't fill immediately and we had to wait for half an hour or so - now it won't fill at all! And the metal button-type flush device on the top can just be pushed without anything happening. I've prised off the top and there is barely any water in the cistern and it isn't refilling at all. The inside parts are plastic and I haven't a clue where to start - and don't want to call in the (expensive) plumbers!
Thanks

Clair
July 2008