If you can blow air through both ways on the NRV, then I strongly suspect you've found the culprit and won't need to go near the pump.
Whilst the NRV is off, take the opportunity to flush the rear pipework by running the pump a few times. If you haven't got some pipe to extend it to the floor, you can run it into the channel and it will run off like rainwater does anyway.
I soaked my NRV in a solution made using a Milton sterilising tablet, but white vinegar or some other household/bathroom descaler may also work.