So looks like the consensus is I used a stupid amount of sealant, left it dry for a few days before adding the oil, allowing the sealant curing process to start eating way at the metal in the gear box?

Thanks all for the advise, I bow my head in shame, consider me scolded and hopefully educated.

Cross fingers the mighty 020 can recover from this abuse or it's off the the wreaking yard to try and find another gearbox. mk2 is out of the question in Brisbane so a mk3 gearbox will have to do.