flow through hot and cold tap non existent?

Increasingly low flow of water through bathroom tap.
Taps are single unit -two taps and single faucet.
Problem has steadily increased to point of zilch for flow.
Kitchen tap starts strong but cuts back a little- downstairs tap slow but flowing.
Shower appears ok but upstairs toilet lacks -often more than one flush
Where do I start ??
All help greatly appreciated

Merry Christmas