Morning Ian,
Rather than "trickle" and if you have one, use an "old school" battery charger and give it a 12v - 13v belt
, for say 30 -60 minutes and then see if the "trickle" charger will work away.
I have had some MotoBatt batteries respond to treatment and I sit the battery outside in case of any accident, there shouldn't be, but better safe than sorry.
If there is no response then I would suggest a replacement is required.
About 2 months ago when Kat and I were out on a run, the Bros suddenly stopped at traffic lights, only a "click" from the solenoid, then not even a "click".
It appeared that one of the terminals had fractured from the guts of the battery, as there was no voltage reading between the terminals. So, internal failure is not unheard of.
Hope you can sort it one way or t'other.
Good health, Bill