How do I stop my Aspen toilet cistern from overflowing into the pan?

I have an Aspen (Close Coupled WC) from Bath Store. Lately when flushed, after flush has stopped water continues to trickle into toilet pan. I have seen lots of questions and answers re Grohe cisterns, which sounds similar but dont really know what I am looking for.
When I have removed the lid, the stack with the float on the left hand side does not seem to rise when the cistern refills.
Any ideas anybody as I keep having to shut the water off at inlet until it needs flushing?