Why didn’t Ocado send me my £18 voucher as promised?

It claimed it had emailed me but the only voucher it sent was for a paltry 33pOcado recently sent me an “£18 money-off voucher” which involved “simply completing three shops by 24 September”. I did so within the deadline but never received the voucher, so telephoned Ocado in October at which time they confirmed that I had completed the requisite number of shops. But the only voucher I received was for 33p and I complained that this was a “hollow” offer w