@ Christopher, you're getting to know me
and you're right, I doubt it'll work this late in the season, but I'm looking forward to you telling me I was wrong - I suppose it depends on whether we get an Indian Summer this year - we certainly deserve one!
I've just been out and looked at my astrantias and there's absolutely no sign of flowers, but as I said these were newly planted late this spring as we're creating a new garden here, starting from scratch. In my last garden, which was north-facing and alternately damp then parched, the astrantias were superb and when we left last August they were in full flower. I lifted and divided them about every three years, and as the garden was 'city dirt' they were mulched with organic farmyard manure every year which seemed to suit them.
You may be right, if they've had a lot of nitrogen-type feed they've probably grown a bit lush and without having had any sun to stimulate flowering .... what about giving them a small dose of tomato fertiliser to encourage flowering?
I'm looking forward to seeing mine flower next summer - that's gardening, already looking ahead 12 months!
I don't know Jumble Hole, but having Googled, it looks gorgeous and is one I'll definitely consider for the future.