Make sure the lid is the right way round - if it isn't it will usually still work but there is no retention.
Otherwise, if you have the small waste container you can replace the top part of the box that has the slide on it. Have a look at
www.leisurespares.co.uk
where you will find a complete breakdown of all the parts along with part numbers. They are based at Boroughbridge, N Yorks and will let you collect by arrangement provided you take the exact cash with you.