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how to stop water overflowing from concealed cistern?

I have toilet appliance (N313253F317) with fill valve (NJ313). According to the instructions an adjustable screw bolt can be used to adjust the water level. The cistern was installed about 2 years ago and the water level was OK, but it has now started overflowing and movement of the adjuster has no affect. Any suggestions?
Don, May 2013