It may not have been a scam in the typical sense, but if not...

It may not have been a scam in the typical sense, but if not then it was certainly director incompetence of the highest level. They were very happy to sign-off on all those quarterly reports showing sales going through the roof and no loan on the balance sheet. Those quarterly reports in hindsight were very obviously not reflecting a true picture of the business and they must have been aware of that at the time. I don't buy their excuse of accounting standards, the error was more than that. I personally still believe it was done on purpose and somebody probably made a lot of money out of this, but it was such a complex web of various handshake deals that it's too hard to prove.

In terms of the write off. You can't do anything in your 18/19 tax return unless you sold you already sold your shares to Delisted. For the 19/20 tax return you will be able to recognise an 80% capital loss. The remaining 20% of shares you still hold. If you you wish to recognise the loss on that final 20% portion then you need to sell them to Delisted, or alternatively just hold onto them and hope for a miracle.