Wonder how many never get cashed due to the hassle
Not sure how much hassle it actually is to be honest - OK annoying if they have closed your bank branch locally - but my bank allows you to pay in cheques via an app on your phone (take photo of cheque), this is Lloyds.
The OP has 6 months to get the cheque and pay it in as well.
Getting Avanti to cancel it and re-issue it might be real aggro. I bet this has happened because something about the bank card it was paid with no longer matches and thus the refund failed. Maybe due tot he OP having moved house.
It does not seem that long ago that you had to handle lots of cheques in your personal life, but obv much rarer now. I recall writing them at supermarket check outs

. Then they got little dot matrix printers at the check out where you gave them the cheque and they printed the sum on it for you, then they handed it back for you to check and sign.