Fisher Paykel Washing Machine won't drain?

A few months ago our Fisher Paykel washing Machine wouldn't drain at the end of the wash cycle. We called a repairman and he came while I was out and fixed the machine in a couple of minutes. He told my wife there was a small stone in the filter which stopped it from pumping.

Now we have the same problem again can anyone tell me where this filter is and how to check it without the need to call the repairman and pay $120!

Thanks