I'm wondering if I could write them a note?!
I'm probably going against the tide, but I would be mortified if someone put something through my door about my pets. If you can have a conversation about them, where you naturally manage to give information, eg. good vets, vaccinations etc, then that would probably work really well. But a note just sounds like you think they aren't very good rabbit owners, to me, whatever the very best of intentions and careful wording.
Oooooo! I love this idea!I would be mortified too...
If you're determined to write a note, I'd suggest you write down the details for your rabbit vet, and say it's "as promised - sorry it took so long to get to you. Hope you (and bun) are settling in ok, and if you need any rabbit advice, you know where to find me". When you next speak to them, say you have a really great rabbit book if they'd like to borrow it.