Hey , I think I managed to inbox you re this but heres my response (links removed as this blog won't let me post links)
Hey Andy
By the sounds of it you have requested and received an ACRO police report? Can I ask did it say 'NO TRACE' or 'NO LIVE TRACE' ? If your offence (Im presuming this is your only criminal office) was from 1984 theres a chance that it isn't even on your criminal record and might just say 'NO TRACE' ...either way you need to also ask ACRO for a 'Subject Access' report, this will give you all the 'Stepped Down' information Canada Immigration will need re the detail and will show you every scrap of information held on you. Heres the link to the ACRO Subject Access request form (you can call them and they'll tell you over the phone by the by but you will still need the report)
'Google ACRO Subject Access request'
If you have already uploaded your Police Report you might find it impossible to upload your Subject Access report, the only way that I found to do this was by raising a 'Case Specific Query' at this link
'Google Canada Immigration case specific query form'
And under 'Type of Application' choose eTA - Case Specific Query...then write them a polite message saying how long its been so far (be very clear about that) and upload the detailed report received from the police. Weirdly enough 2 weeks after I did exactly this I got my approval through!
I guess the bit that must really bug you is that under Canada Immigration rules you can actually class yourself as 'Deemed Rehabilitated' (its a self cert sort of thing) as your crime is over 10 years old and as such you could get through Canada customs perfectly legally, its just this eTA form doesn't allow that, giant pain in the derriere!
Good luck, come back to me when you get stuck
