It's very easy to replace the washers and a set of mixed sized washers are available at B&Q for about 3 quid. Turn the water off at the stop **** then flush the loo once to empty the cistern.
Here's how to do the old brass type.
http://diydata.com/plumbing/ball_****_repair/b all_valve_repair.php
The more modern plastic type is very similar. It is self evident when you get it apart. Remember to clean off and scale or scummy bits before replacement. around the area wher the washer seats.
Shoulkd save you a fortune on a plumbers costs and will take about 20 mins to do.