Thanks, Chris, I had no idea Known now has so many community plugins!
I installed the Reactions plugin but when I enabled it I got a slug of warning messages about headers etc. so I've had to disable it again. Oh well!
Chris, as an #indieweb end user I've never been able to work out how to do what you've done here - i.e. "like" a post made on another site on your own. I'm sure I'm being a numpty, but any tips on how to do this would be much appreciated!