Were the Jill books the ones with the sisters called April, May and June, who were terrible riders but amiable? (And in either case, why does that stick in my head?)

Yes! Although I cannot answer for the second query. Jane Badger's excellent horse book site informs me they are the daughters of Captain Cholly-Sawcutt (gosh).

(Also glad you're okay after the NZ earthquakes news earlier in the week)

Thank you! I didn't feel them (although in hindsight I did wake up quite a few times in the night) and am quite a few hills away from the coast but (as you know) it's always a bit unnerving to be reminded of what else is out there.