I actually pulled the entire bracket out, but even with much more freedom to move the accumulator canister around, I still had no luck. It's just millimeters away from clicking home, I know it is, but I can't get enough force onto it. I hoped that a crowsfoot socket on the end of some long extensions would fit around the hose and give me something to push against, but the biggest socket I have - 19mm - isn't quite big enough.
Just one of those unbelievable frustrations. If it had clicked in, the rest of the system would have been connected up in about 10 minutes.
@SAR-Swimmer - thanks for the reassurance
Accumulators aren't as cheap or readily available here as they are in the USA, so replacement is a bigger deal.