Just an update for information, in case anyone else is facing this same problem.
I applied for an eTA again this year, sending through a copy of my police certificate but also a copy of my CLEAN DBS certificate (disclosure scotland cert), and a long letter pointing out the relevent parts of canadian and UK legislation, but twice it got knocked back, even though my offence in canadian law amounts to "an offence which has a maximum penalty of UP TO 10 years imprisonment".
The border officials kept rejecting my eTA under the reason that I have commited an offence which " has a maximum penalty of AT LEAST 10 years imprisonment". This is not correct, but as I reapplied for the eTA (you can do this as many times as you wish, paying the $7 each time and each application will be assessed on its own merit), they immediately knocked it back again with the same reason.
It seems that the eTA border officials are trained to be cautious and not look into the legislation. So I think my only option will be to hire a immigration lawyer and/or apply for a full visa in order to fully explain my situation. Applying for a visa costs $1000CAD so it's not something that I'm trivially going to try my luck with just now.