my external overflow is pouring water?

I had new ball valve replaced in my cistern about 18months ago, now the same problem has occured, water is pouring out of my extenal overflow, i have tried to sort the problem out, but flushing the loo and running water, but it has not cleared the problem, is there anything i could do to the interior of the cistern to try and sort the problem out? someone suggested we may need a new filter? where is the filter situated? thanks for any help you may be able to give, Sally x